# LinearPairedArcDisplay Auto-generated @jbrowse/mobx-state-tree API for the current JBrowse release — see [pluggable elements](https://jbrowse.org/jb2-staging/docs/developer_guide/) for concepts. Provided by the `arc` plugin. [View source](https://github.com/GMOD/jbrowse-components/blob/main/plugins/arc/src/LinearPairedArcDisplay/model.ts). ## Example usage Selected on a `VariantTrack` of structural variants: each feature draws an arc from its position to its mate breakend, even when the mate is on another chromosome / displayed region. Short ticks mark each breakend's mate direction; clicking an arc opens the variant details: ```js { type: 'VariantTrack', trackId: 'sv', name: 'Structural variants', assemblyNames: ['hg38'], adapter: { type: 'VcfTabixAdapter', uri: 'https://example.com/sv.vcf.gz', }, displays: [ { type: 'LinearPairedArcDisplay', displayId: 'sv-LinearPairedArcDisplay', }, ], } ``` a non-block-based display that draws one arc per feature from its position to its mate breakend (parsed from the VCF `ALT`), connecting the two loci of a structural variant even across displayed regions / chromosomes; rendered as plain SVG on the main thread. For arcs that span a single feature's own start–end use [LinearArcDisplay](../lineararcdisplay) instead. The configuration slots for this model are documented on its [config schema page](../../config/linearpairedarcdisplay). Members a composed model contributes are listed here too, so these tables are the whole surface. ## Properties | Member | Description | Defined by | | --- | --- | --- | | **type**
type: types.literal('LinearPairedArcDisplay') | | LinearPairedArcDisplay | | **configuration**
configuration: ConfigurationReference(configSchema) | | LinearPairedArcDisplay | | **id**
id: ElementId | | [BaseDisplay](../basedisplay#property-id) | ## Volatiles | Member | Description | Defined by | | --- | --- | --- | | **error**
error: undefined as unknown | | [BaseDisplay](../basedisplay#volatile-error) | | **statusMessage**
statusMessage: undefined as string | undefined | | [BaseDisplay](../basedisplay#volatile-statusmessage) | | **statusProgress**
statusProgress: undefined as number | undefined | determinate progress fraction [0,1] for the current status, or undefined when the in-flight phase is indeterminate. Set alongside `statusMessage` by `setStatusMessage`; a display that never shows a bar simply leaves it undefined. | [BaseDisplay](../basedisplay#volatile-statusprogress) | | **scrollTop**
scrollTop: 0 | | [TrackHeightMixin](../trackheightmixin#volatile-scrolltop) | | **features**
features: undefined as Feature[] | undefined | | [ArcFetchModel](../arcfetchmodel#volatile-features) | | **loadedFetchSignature**
loadedFetchSignature: undefined as string | undefined | `fetchSignature` as it stood when the held data was committed — the loaded half of this family's freshness compare. Written only by `commitFetchResult` and cleared only by `reload`, so a display cannot stamp data it did not fetch or forget to invalidate on retry; the data itself stays display-owned (arc keeps stale arcs on screen under the loading overlay, HiC keeps the stale matrix). | [GlobalFetchMixin](../globalfetchmixin#volatile-loadedfetchsignature) | | **forceLoadTrack**
forceLoadTrack: false | The force-load button's track-wide approval. Volatile so it never reaches a saved session; the `forceLoad` config slot is the durable form. | [RegionTooLargeMixin](../regiontoolargemixin#volatile-forceloadtrack) | | **byteEstimate**
byteEstimate: undefined as ByteEstimate | undefined | The last byte measurement: bytes, the span they were taken at, and whether zooming has been shown not to shrink them. Survives `clearAllRpcData`; dropped on chromosome navigation and on a tier swap. | [RegionTooLargeMixin](../regiontoolargemixin#volatile-byteestimate) | | **gateMeasuredViewportKey**
gateMeasuredViewportKey: undefined as string | undefined | The viewport key the gate last asked the adapter about, on either axis. Separate from `byteEstimate` because a density refusal measures no bytes. | [RegionTooLargeMixin](../regiontoolargemixin#volatile-gatemeasuredviewportkey) | | **canvasDrawn**
canvasDrawn: false | flips true on first paint; read by test selectors to detect render | [RenderLifecycleMixin](../renderlifecyclemixin#volatile-canvasdrawn) | | **currentRenderingBackend**
currentRenderingBackend: undefined | current backend reference, updated on context-loss recovery. Typed `unknown` (not generic `B`) on purpose: this mixin is composed by every display via a non-generic factory, so the per-display backend type `B` isn't known here — it's supplied at `attachRenderingBackend` and narrowed with `as B` inside the autoruns. Don't "fix" the cast. | [RenderLifecycleMixin](../renderlifecyclemixin#volatile-currentrenderingbackend) | | **renderTick**
renderTick: 0 | counter the render autorun observes; bumped to force a re-render | [RenderLifecycleMixin](../renderlifecyclemixin#volatile-rendertick) | | **autorunsInstalled**
autorunsInstalled: false | guards attachRenderingBackend so the autorun pair spawns once per instance | [RenderLifecycleMixin](../renderlifecyclemixin#volatile-autorunsinstalled) | | **renderError**
renderError: undefined | the render-backend (GPU/Canvas2D init or context-loss) error, or undefined. Single source of truth for the render-error terminal state: `useRenderingBackend` writes it from the canvas-init mechanism so the model — not React-local hook state — owns every terminal state. Read by `displayPhase` (whose `renderError` term outranks `loading`, suppressing the scrim) and by `DisplayChrome` (shows the retry overlay). | [RenderLifecycleMixin](../renderlifecyclemixin#volatile-rendererror) | | **activeStopToken**
activeStopToken: undefined as StopToken | undefined | stop token of the in-flight fetch, or undefined when idle | [FetchMixin](../fetchmixin#volatile-activestoptoken) | | **fetchGeneration**
fetchGeneration: 0 | bumps at every fetch end; autoruns read it to re-evaluate, and it doubles as the staleness epoch inside runFetch | [FetchMixin](../fetchmixin#volatile-fetchgeneration) | | **reloadCounter**
reloadCounter: 0 | Bumped by `reload()` and read unconditionally by the fetch autoruns, so a user retry re-runs the body even where nothing else moved — after an error every other fetch input is unchanged. It is also the half that survives a `reload()` override that forgets to invalidate, which is the dead Retry button `makeRetryContractCheck` reports. Declared here because this is the one mixin both LGV fetch foundations compose, the same argument that put `fetchInert` below; the comparative family carries its own on `SyntenyFetchStateMixin` (ADR-054). | [FetchMixin](../fetchmixin#volatile-reloadcounter) | | **statusWindow**
statusWindow: createStatusWindow(writeStatus(self)) | This display's status field, and the only thing that writes it: one throttle window, one slot per concurrent operation, so N parallel per-region fetches thin to one stream between them rather than N and a second operation cannot end the first one's label (ADR-081). Lent whole to `createStopTokenRotation` by a display that also runs a bare-autorun fetch — see `StatusReporter`. | [FetchMixin](../fetchmixin#volatile-statuswindow) | | **fetchCanceled**
fetchCanceled: false | true after the user explicitly cancels a load (the loading overlay's cancel button → `cancelFetchByUser`). A durable, blocking state — unlike `cancelFetch`, it does not retrigger the fetch autoruns — so the load stays stopped until the user retries (`reload`) or the viewport changes. Any new fetch clears it (`runFetch` resets it at the start). | [FetchMixin](../fetchmixin#volatile-fetchcanceled) | | **fetchRotation**
fetchRotation: createStopTokenRotation(self, { statusWindow: self.statusWindow, })
| **The latest-wins machine this mixin is a wrapper around**, and not a second one: `createStopTokenRotation` owns token rotation, the `isCurrent` guard, the status slot and the supersede-versus-end rule (ADR-080, ADR-081), for every fetch in the codebase that has one. `runFetch` adds the observable bookkeeping a display needs on top — `isLoading`, `error`, `fetchGeneration`, `fetchCanceled` — and nothing else.

It was two implementations of that machine until 2026-08-20, which is how they came to disagree about whether a completed fetch releases its token. A display's *primary* fetch is this wrapper; a second concurrent fetch on the same node holds a rotation of its own, which is why the primitive is the thing that exists and this is the thing built on it (ADR-054 §1).

It is lent this display's `statusWindow`, so the fetch takes a slot on the one field rather than opening a second window over it — the whole point of `StatusReporter`. | [FetchMixin](../fetchmixin#volatile-fetchrotation) | ## Getters | Member | Description | Defined by | | --- | --- | --- | | **conf**
ModelInstanceTypeProps<Record<…>> & { setSubschema(slotName: string, data: Record<string, unknown>): any; setSlot(slotName: string, value: unknown): void; } & IStateTreeNode<...>
| the config typed off the concrete schema; `ConfigurationReference` erases `self.configuration` to `any`, so reads route through this to stay typed (same move as `BaseAdapter`) | LinearPairedArcDisplay | | **lineWidth**
number | arc stroke width in px, from the promotable `lineWidth` slot (track-menu slider writes it); flat across all arcs | LinearPairedArcDisplay | | **minScore**
number | arcs whose feature scores below this are not drawn; 0 (the default) draws every arc, as does any feature carrying no score | LinearPairedArcDisplay | | **scoreRange**
{ min: number; max: number; } | undefined | the score span the filter slider is laid out over, `undefined` when the loaded features give it nothing to filter on | LinearPairedArcDisplay | | **arcStyles**
{ k1: {…}; k2: { refName: string; start: number; end: number; mateDirection?: number | undefined; }; feature: Feature; alt: string | undefined; color: string; }[] | undefined
| per-arc styling and endpoint pairs (one per ALT), evaluated once when features/config change. Keeps the color jexl and makeFeaturePair (which runs parseSvAlt) out of the per-pan render loop. Deduped on a canonical endpoint-pair key: a paired feature is emitted from both endpoints and reciprocal BNDs arrive as two records, so the same arc otherwise draws twice whenever both endpoints are in the fetched regions. | LinearPairedArcDisplay | | **parentTrack**
AbstractTrackModel | | [BaseDisplay](../basedisplay#getter-parenttrack) | | **RenderingComponent**
FC<…> | | [BaseDisplay](../basedisplay#getter-renderingcomponent) | | **DisplayBlurb**
FC<{ model: ModelInstanceTypeProps<{ id: IOptionalIType<ISimpleType<string>, [undefined]>; type: ISimpleType<string>; }> & { ...; } & { ...; } & IStateTreeNode<...>; }> | null
| | [BaseDisplay](../basedisplay#getter-displayblurb) | | **adapterConfig**
Record<string, unknown> | | [BaseDisplay](../basedisplay#getter-adapterconfig) | | **isMinimized**
boolean | Returns true if the parent track is minimized. Used to skip expensive operations like autoruns when track is not visible. | [BaseDisplay](../basedisplay#getter-isminimized) | | **hoveredFeature**
unknown | Overridable hook (default `undefined`): what the pointer is currently over, for readers **outside** the display. `LinearGenomeViewContainer` publishes it to `session.hovered`, the view-wide "what is the user pointing at" channel a plugin can subscribe to.

Declared here because a cross-display consumer can only read a name the base declares — the same reason `SyntenyFetchStateMixin.fetchInert` is a hook rather than a getter each display invents. The container used to read `featureUnderMouse`, which only the wiggle, alignments and Manhattan families spelled that way — canvas said `hoveredFeature`, variants `hoveredGenotype` — so the channel carried a hover from a third of the display types and nothing said which. It also asked only `displays[0]` of each track.

`unknown` because the payload genuinely differs — a read, a wiggle bin, a SNP, a genotype cell — and `session.hovered` is typed to match ("can be anything; code that wants to deal with this should examine it"). Narrow it in the override. | [BaseDisplay](../basedisplay#getter-hoveredfeature) | | **featureNoun**
string | Overridable hook (default `'feature'`): the SINGULAR word for one of the things this display draws, as a menu row or a chip says it — "Hide this read", "Showing 3 variants".

Declared here for the same reason as `hoveredFeature` above: it is read across the display boundary, by chrome that has no idea which display it is drawing for (`SoloSelectionChip`, alignments' group-label overlay), and a name only the base declares is a name every such consumer can rely on. Two displays declared it independently and one of those declarations WAS this default.

**A control keeps the generic word; content takes this one.** "Variant height" reads as a different setting from "Feature height" when it is the same one, so the shared menus stay on "feature" however the display answers here, and the noun varies where it names what the user is looking at — "Showing 3 variants", "Hide this read". A display drawing something the generic word already fits is right to leave this alone.

Distinct from the per-hit noun a context menu takes off the clicked item's own `type` ("mRNA", "gene"); that names one annotation, this names what the track holds. The hit noun falls back to this. | [BaseDisplay](../basedisplay#getter-featurenoun) | | **featureWidgetType**
{ type: string; id: string; } | Overridable hook: which widget `openFeatureWidget` opens for one of this display's features. The default is the generic one, which is what a display drawing plain features wants and what the canvas base spelled out by hand.

An override is a display whose features have a vocabulary of their own — a read, a variant, a synteny block — and the `id` is deliberately part of it: two displays naming one id share the drawer panel, which is the behaviour when the two are showing the same kind of thing. | [BaseDisplay](../basedisplay#getter-featurewidgettype) | | **height**
number | | [TrackHeightMixin](../trackheightmixin#getter-height) | | **resizing**
boolean | True for the duration of a height drag on this track, whichever handle is running it. A display whose row geometry is a function of the track height restretches every row per animation frame, and can use this to sit an expensive per-frame layer out of the drag (MAF's dense per-base letter overlay is a Canvas2D pass that scales with rows x columns).

The flag itself is the track's (`BaseTrackModel`), so the view brackets a drag without needing the active display to have opted into this mixin. Reading it here is what makes `self.resizing` available to a display that did. | [TrackHeightMixin](../trackheightmixin#getter-resizing) | | **scrollableHeight**
number | Overridable hook: how far this display's content can scroll past its viewport, in px. `Infinity` (the default) means "this display doesn't scroll internally" — `setScrollTop` then never clamps and the re-clamp autorun below is inert, so a non-scrolling display pays nothing and, crucially, never evaluates a getter that would read view geometry.

A display that scrolls a canvas overrides this with `max(0, contentHeight - viewportHeight)`, and gets the clamped setter plus the shrink autorun for free. It is the single "does it scroll, and by how much" answer: the wheel handler (`useVirtualScrollWheel`) and `VerticalScrollbar` read the same getter. | [TrackHeightMixin](../trackheightmixin#getter-scrollableheight) | | **gateEnabled**
boolean | | [ArcFetchModel](../arcfetchmodel#getter-gateenabled) | | **view**
ModelInstanceTypeProps<_OverrideProps<_OverrideProps<…>, { ...; }>> & ... 25 more ... & IStateTreeNode<...>
| | [ArcFetchModel](../arcfetchmodel#getter-view) | | **canvasWidth**
number | The box the arcs are laid out in: the on-screen `

Not the `canvasWidth` an SVG body gets from `renderDisplaySvg`, which is the viewport width the shell paints at — see `renderArcSvg`. | [ArcFetchModel](../arcfetchmodel#getter-canvaswidth) | | **viewSignature**
string | undefined | Arc's half of `GlobalFetchMixin`'s freshness compare: the static-block set is the whole staleness axis (there is no `loadedRegions` spatial map — every feature is fetched into one array), so panning or zooming past a block boundary refetches and a scroll inside the loaded blocks does not. | [ArcFetchModel](../arcfetchmodel#getter-viewsignature) | | **displayPhase**
DisplayStatusPhase | Narrows the foundation's `displayPhase` to the backend-free variant. Arc composes the render lifecycle with the rest of the foundation but never calls `attachRenderingBackend` — it paints JSX ``s, on screen and in SVG export alike — so `renderError` is a phase it cannot reach, and the narrower type is what lets `DisplayStatusChrome` (whose banners have no backend `retry()` to offer) accept this display with neither a cast nor a dead branch. On the model rather than derived inside `BaseDisplayComponent` so the component can't disagree with the model, and so arc publishes `data-display-phase` for tests like every other display.

`isLoadingOrCanceled`, never a bare `isLoading` — see that getter. | [ArcFetchModel](../arcfetchmodel#getter-displayphase) | | **painted**
boolean | Arc's first-paint signal, overriding `RenderLifecycleMixin`'s `painted`: nothing ever flips `canvasDrawn` here, because arc attaches no rendering backend, so the data arriving is the signal. Stays true across a refetch so `data-display-drawn` and the loading anti-flash don't churn on pan; the stricter, staleness-aware `svgReady` is the export gate.

On the model rather than derived in `BaseDisplayComponent`, for the same reason `displayPhase` is: a component-side derivation is free to disagree with what the model believes, and this one feeds an attribute (`data-display-drawn`) that the screenshot and browser harnesses wait on. | [ArcFetchModel](../arcfetchmodel#getter-painted) | | **host**
RegionHost | The hosting view as the `RegionHost` contract — see `containingHost` for the cast it owns, why the name is `host` and not `view`, and why both foundations still declare the name over one body. | [GlobalFetchMixin](../globalfetchmixin#getter-host) | | **viewportEmpty**
boolean | No content block is on screen, so this display has nothing to fetch and nothing to paint — see `viewportEmpty.ts` for the one viewport that reaches it, how narrow that is, and why the state still has to be terminal rather than a permanent scrim. Both foundations declare it over that one expression, the same way they each declare `host` and `paintInert`. | [GlobalFetchMixin](../globalfetchmixin#getter-viewportempty) | | **canRender**
boolean | Overrides `RenderLifecycleMixin`'s default-true hook with the LGV precondition both foundations share — see `foundationCanRender`. | [GlobalFetchMixin](../globalfetchmixin#getter-canrender) | | **fetchSignature**
string | undefined | Signature of the fetch the current view and settings call for — the display's `viewSignature` plus the serialized `rpcProps()` axis. What `runGlobalFetch` gates on, captures at issue, and stamps at commit. | [GlobalFetchMixin](../globalfetchmixin#getter-fetchsignature) | | **dataCurrent**
boolean | The shared freshness answer, now derived rather than a hook: data has been committed (`loadedFetchSignature` is only ever written beside it) and it was fetched for the current view and settings. A pan inside the loaded blocks stays current; a block entering, a tier step, a settings change or a `reload()` moves one side of the compare and refetches. | [GlobalFetchMixin](../globalfetchmixin#getter-datacurrent) | | **paintInert**
boolean | Fills `RenderLifecycleMixin`'s `paintInert` hook — see there for why a failed fetch has to read as finished to the consumers outside the display, and `foundationPaintInert` for the second such state and why both fetch families answer it through one function. Overridable, as the hook is: a display with a third inert state of its own says so here. | [GlobalFetchMixin](../globalfetchmixin#getter-paintinert) | | **svgReady**
boolean | Policy single-sourced in `computeSvgReady`; this family supplies only the freshness half, which `foundationSvgReady` reads as `dataCurrent` or the vacuous currency of `viewportEmpty`. Note it requires the dataset to actually be current, NOT merely "not currently fetching": the fetch trigger is a debounced `afterAttach` autorun, so at export time `isLoading` can still be false with no data yet — a `displayPhase !== 'loading'` test would then capture an empty render. Never gates on `canvasDrawn`, which an off-screen export never sets. Off-screen renderers gate on it via `awaitSvgReady(model)`. | [GlobalFetchMixin](../globalfetchmixin#getter-svgready) | | **densityGateEnabled**
boolean | Whether the density axis applies. `CanvasFeatureGateMixin` contributes `true` beside its measurement; byte-only displays leave it. | [RegionTooLargeMixin](../regiontoolargemixin#getter-densitygateenabled) | | **byteGateAdapterConfig**
Record<string, unknown> | The adapter config the gate measures — the one at `byteGateAdapterPath`. Overridable for a display whose adapter config is synthesized rather than read off the track. | [RegionTooLargeMixin](../regiontoolargemixin#getter-bytegateadapterconfig) | | **configuredFetchSizeLimit**
number | The display's `fetchSizeLimit` slot, from `regionTooLargeConfigSchemaFields`. | [RegionTooLargeMixin](../regiontoolargemixin#getter-configuredfetchsizelimit) | | **densityTooLarge**
boolean | The density axis's verdict; canvas overrides it. | [RegionTooLargeMixin](../regiontoolargemixin#getter-densitytoolarge) | | **byteGateAdapterPath**
string[] | Where on the track config the measured adapter sits. A tiered display overrides this one hook (MAF: `['adapter', 'summaryAdapter']` while `showSummary`), and both the measurement and the budget follow it. | [RegionTooLargeMixin](../regiontoolargemixin#getter-bytegateadapterpath) | | **adapterFetchSizeLimit**
number | undefined | The measured adapter's own `fetchSizeLimit` slot, read off the live track config rather than the `adapterConfig` snapshot, which omits slots at their default. | [RegionTooLargeMixin](../regiontoolargemixin#getter-adapterfetchsizelimit) | | **configForceLoad**
boolean | The declarative `forceLoad` slot. | [RegionTooLargeMixin](../regiontoolargemixin#getter-configforceload) | | **gateViewport**
GateViewport | undefined | What a measurement taken now would be about: the span on screen and a key for the stretch of genome it covers. Undefined until the view is measured, and the mixin's only read of the view. Captured before the fetch's round trip, never at commit. | [RegionTooLargeMixin](../regiontoolargemixin#getter-gateviewport) | | **byteGateAdapterKey**
string | Which tier the estimate is about, as a comparable string. | [RegionTooLargeMixin](../regiontoolargemixin#getter-bytegateadapterkey) | | **aboveForceLoadFloor**
boolean | Whether the span on screen is at or above `AUTO_FORCE_LOAD_BP`, the one comparison against that constant. False on an unmeasured view. | [RegionTooLargeMixin](../regiontoolargemixin#getter-aboveforceloadfloor) | | **gateExempt**
boolean | Nothing may gate on either axis: the `forceLoad` slot or the button. | [RegionTooLargeMixin](../regiontoolargemixin#getter-gateexempt) | | **estimatedFetchBytes**
number | undefined | The stored estimate's bytes; undefined when nothing has been measured. | [RegionTooLargeMixin](../regiontoolargemixin#getter-estimatedfetchbytes) | | **gateMeasurementStale**
boolean | Whether the last measurement is about a viewport the user has since left. True before any measurement. | [RegionTooLargeMixin](../regiontoolargemixin#getter-gatemeasurementstale) | | **gateByteLimit**
number | The byte budget: the adapter's limit, else the display's, doubled below `AUTO_FORCE_LOAD_BP`. Read only through `resolvedByteLimit()`. | [RegionTooLargeMixin](../regiontoolargemixin#getter-gatebytelimit) | | **gateActive**
boolean | Whether the gate may act right now, on any axis: opted in, not exempt, view measured. The view is read last, so an ungated display never touches it. | [RegionTooLargeMixin](../regiontoolargemixin#getter-gateactive) | | **densityGateActive**
boolean | `gateActive` plus the density axis's own terms: the axis is on, and the span is above the floor. | [RegionTooLargeMixin](../regiontoolargemixin#getter-densitygateactive) | | **tooLargeStatus**
RegionTooLargeStatus | The verdict and its banner text, from the stored estimate against `resolvedByteLimit()` and the density axis when it may act. | [RegionTooLargeMixin](../regiontoolargemixin#getter-toolargestatus) | | **regionTooLarge**
boolean | | [RegionTooLargeMixin](../regiontoolargemixin#getter-regiontoolarge) | | **regionTooLargeReason**
string | Banner text for the axis that tripped; empty when not too large. | [RegionTooLargeMixin](../regiontoolargemixin#getter-regiontoolargereason) | | **zoomCanReleaseGate**
boolean | Whether "zoom in to see features" is honest advice. Density always releases on zoom; bytes only if the last zoom-in moved the estimate. | [RegionTooLargeMixin](../regiontoolargemixin#getter-zoomcanreleasegate) | | **gateSkipsMeasuredViewport**
boolean | The skip both fetch skeletons apply: the banner is up and its measurement already describes the viewport on screen. | [RegionTooLargeMixin](../regiontoolargemixin#getter-gateskipsmeasuredviewport) | | **rendersCanvas**
boolean | Overridable hook (default true): whether this display paints a canvas in its **current** configuration, as opposed to a deliberate static placeholder (LD with the triangle off, sequence past base resolution — both render a message where the ` | [RenderLifecycleMixin](../renderlifecyclemixin#getter-renderscanvas) | | **isLoading**
boolean | true while a fetch is active | [FetchMixin](../fetchmixin#getter-isloading) | | **isLoadingOrCanceled**
boolean | `isLoading` widened to cover a user-canceled load. **This, not `isLoading`, is what a `displayPhase` loading term wants.** `cancelFetchByUser` clears the stop token synchronously, so `isLoading` goes false the instant the user clicks Cancel — and the loading overlay that unmounts on it is carrying the Retry button, which is the only way back: the state is deliberately durable, so no autorun restarts the fetch on its own. A bare `isLoading` therefore reads as `ready` over a display that is stopped, empty and offering nothing.

Arc read `isLoading` directly and had exactly that hole. It is a getter here so no family has to remember the second term. | [FetchMixin](../fetchmixin#getter-isloadingorcanceled) | | **fetchInert**
boolean | Overridable hook (default false): the states where this display deliberately never fetches, so it holds no data and none is coming. Sequence sets it past base resolution ("Zoom in to see sequence"); LD sets it with the triangle toggled off.

**One hook, three readers**, and that is the whole point — a display that grows such a state has one thing to say rather than three, and the reader it would have forgotten is always the one outside itself:

- the loading scrim (`computeLoadingTerm`), which otherwise parks over the placeholder, permanently once a cancel has been clicked; - the SVG export (`computeSvgReady`'s `extraTerminal`), whose `awaitSvgReady` is an unbounded `when`, so one such display hangs the whole view's export; - the dev-only retry check (`makeRetryContractCheck`), which would otherwise report a dead Retry on a display correctly declining to load anything.

It was three hooks — `loadingSuppressed`, `svgReadyExtraTerminal` on each of the two foundations, and `fetchInert` on the comparative family, which had already collapsed them. Both LGV displays that override it returned one expression for all three, and one of the three was hard-coded `false` on the global family for a while, which is how LD came to be able to express only half its own state. Same name and same meaning as `SyntenyFetchStateMixin.fetchInert` now, so the retry check reads one field across all three fetch families. ADR-082.

A hook rather than a `displayPhase` override, because overriding the getter means restating the whole loading condition — which is how sequence came to hold a verbatim copy of the other terms, one `git blame` away from silently missing the next one added.

It lives **here** because this is the one mixin all three display foundations compose. Same argument, one level down, that put `rendersCanvas` on `RenderLifecycleMixin` beside `canvasDrawn`. | [FetchMixin](../fetchmixin#getter-fetchinert) | | **awaitingPrerequisite**
boolean | Overridable hook (default false), read only by the dev-only retry check (`makeRetryContractCheck`): "this run declined because a prerequisite fetch in another autorun has not landed, and its arrival wakes this one again". It **defers** the retry verdict to that later run rather than waiving it, so a display cannot spend its retry on a decline it called preliminary.

Two displays say it, one per fetch foundation, which is why it lives beside `fetchInert` rather than on either: HiC's contacts fetch declines until `CoreGetInfo` lands, and `MultiSampleVariantBaseModel`'s `fetchNeeded` declines until `sourcesBase` does. Both have a `reload()` that wakes the prerequisite's autorun as well as their own.

**It has to be strictly narrower than the gate it explains.** One that restates the gate's negation makes every decline a deferred one, so no run is ever judged and the display has silently opted out — an exemption by another name. HiC is in that shape deliberately, because its gate and its prerequisite are one condition; what covers its retry instead is `LinearHicDisplay/infoFetchFailure.test.ts`.

Not for a display deliberately not fetching at all — that is `fetchInert` above, which the loading scrim and the export read too. | [FetchMixin](../fetchmixin#getter-awaitingprerequisite) | | **rpcPropsCacheKey**
string | The RPC cache key both fetch foundations invalidate on: this display's `rpcProps()` payload serialized to a string. `serializeRpcProps` owns the why, including the silently-dead-axis corollary.

Here, beside the two hooks above, for the same reason they are: it describes the display, and every foundation composes this mixin. The per-region family watches it from `SettingsInvalidate` and the global one from its fetch autorun's trigger list — one getter and one name, so the two cannot come to invalidate on different axes. The global side built its own local `computed` over the same function until 2026-08, which was the same value under a second spelling. | [FetchMixin](../fetchmixin#getter-rpcpropscachekey) | ## Methods | Member | Description | Defined by | | --- | --- | --- | | **trackMenuItems**
() => MenuItem[] | | LinearPairedArcDisplay | | **renderingProps**
() => { displayModel: ModelInstanceTypeProps<{ id: IOptionalIType<…>; type: ISimpleType<string>; }> & { ...; } & { ...; } & { ...; } & IStateTreeNode<...>; }
| props passed to the renderer's React "Rendering" component. these are client-side only and never sent to the worker. includes displayModel and callbacks | [BaseDisplay](../basedisplay#method-renderingprops) | | **resolvedByteLimit**
() => number | undefined | The budget the worker enforces and the banner compares against — the one spelling of that pair. Undefined when the gate may not act. | [RegionTooLargeMixin](../regiontoolargemixin#method-resolvedbytelimit) | | **gateFetchState**
() => GateFetchState | The gate as it stands for a fetch about to be issued. Calling it is the capture, which is why it is a method. | [RegionTooLargeMixin](../regiontoolargemixin#method-gatefetchstate) | ## Actions | Member | Description | Defined by | | --- | --- | --- | | **selectFeature**
(feature: Feature) => void | | LinearPairedArcDisplay | | **setLineWidth**
(n?: number | undefined) => void | set arc stroke width; `undefined` resets to the config-slot default | LinearPairedArcDisplay | | **setMinScore**
(score: number) => void | | LinearPairedArcDisplay | | **renderSvg**
(_opts?: ExportSvgDisplayOptions | undefined) => Promise<ReactNode>
| | LinearPairedArcDisplay | | **setStatusMessage**
(status?: RpcStatus | undefined) => void | | [BaseDisplay](../basedisplay#action-setstatusmessage) | | **setError**
(error?: unknown) => void | | [BaseDisplay](../basedisplay#action-seterror) | | **clearHoveredFeature**
() => void | Overridable hook (default no-op): drop whatever `hoveredFeature` reports. The writing twin of that getter, and what `installClearHoverOnViewportChange` calls.

A display that STORES its hover owes an override; one that derives it from the live pointer (MAF, Hi-C, LD) owes nothing, and the default costs it nothing. Declared here so the clear can be installed for every display rather than remembered per display — forgetting it is the failure ARCHITECTURE.md's stored-hover section is about, and it used to be six closures at six call sites, which is six chances to omit one. | [BaseDisplay](../basedisplay#action-clearhoveredfeature) | | **reload**
() => void | base display reload does nothing, see specialized displays for details | [BaseDisplay](../basedisplay#action-reload) | | **setScrollTop**
(scrollTop: number) => void | Clamped into `[0, scrollableHeight]`, so no caller has to remember the bound. Unbounded for a display that leaves `scrollableHeight` at its `Infinity` default. | [TrackHeightMixin](../trackheightmixin#action-setscrolltop) | | **setHeight**
(displayHeight: number) => number | | [TrackHeightMixin](../trackheightmixin#action-setheight) | | **resizeHeight**
(distance: number) => number | | [TrackHeightMixin](../trackheightmixin#action-resizeheight) | | **expandToContentHeight**
() => number | Grow the track by exactly the content it is currently hiding, so a display scrolled over a taller stack ends up showing all of it. The track's resize handle runs this on a double click.

`scrollableHeight` is the whole measurement — it is already every scrolling display's answer to "how much is off the bottom", so no display has to supply a second one. A display that doesn't scroll internally leaves it at `Infinity` and gets a no-op, as does one already showing everything (0).

Routed through `resizeHeight` rather than `setHeight` so grow mode's override still gets to leave grow first; going straight to the slot would let the reactive height re-derive `grownHeight` and the double click would appear to do nothing. | [TrackHeightMixin](../trackheightmixin#action-expandtocontentheight) | | **setFeatures**
(f: Feature[]) => void | `runGlobalFetch` stamps the signature these were fetched for (`GlobalFetchMixin.commitFetchResult`) in the same transaction. | [ArcFetchModel](../arcfetchmodel#action-setfeatures) | | **commitFetchResult**
(commit: () => void, signature: string) => void | The commit half of `runGlobalFetch`: run the display's own store in the same transaction as the signature stamp, so no observer can see fresh data under a stale signature or the reverse. Being the only writer of `loadedFetchSignature` is what makes `dataCurrent` derivable — a display cannot commit without stamping. | [GlobalFetchMixin](../globalfetchmixin#action-commitfetchresult) | | **setByteEstimate**
(measurement: { bytes: number; viewport: GateViewport; }) => void
| The bytes half of a measurement alone, for a test staging a display. Production commits through `commitFetchBytes`. | [RegionTooLargeMixin](../regiontoolargemixin#action-setbyteestimate) | | **clearByteEstimate**
() => void | Drops the estimate and the viewport stamp. `forceLoadTrack` survives: it is a track-wide approval. | [RegionTooLargeMixin](../regiontoolargemixin#action-clearbyteestimate) | | **setForceLoadTrack**
(flag: boolean) => void | | [RegionTooLargeMixin](../regiontoolargemixin#action-setforceloadtrack) | | **commitFetchBytes**
(perRegionBytes: (number | undefined)[], issued: GateFetchState) => void
| The byte axis of a finished fetch, called by the fetch runners with the `gateFetchState()` they captured at issue. Commits the per-region max; an empty batch, or an ungated display, commits nothing. | [RegionTooLargeMixin](../regiontoolargemixin#action-commitfetchbytes) | | **forceLoad**
() => void | The banner's button: exempt the track on both axes and refetch. | [RegionTooLargeMixin](../regiontoolargemixin#action-forceload) | | **markCanvasDrawn**
() => void | | [RenderLifecycleMixin](../renderlifecyclemixin#action-markcanvasdrawn) | | **resetCanvasDrawn**
() => void | | [RenderLifecycleMixin](../renderlifecyclemixin#action-resetcanvasdrawn) | | **stopRenderingBackend**
() => void | | [RenderLifecycleMixin](../renderlifecyclemixin#action-stoprenderingbackend) | | **renderNow**
() => void | | [RenderLifecycleMixin](../renderlifecyclemixin#action-rendernow) | | **setRenderError**
(error: unknown) => void | set/clear the render-backend error. Called by `useRenderingBackend`: with the error when the canvas factory rejects (or context-loss re-init fails), and with `undefined` on successful (re)init and on retry. | [RenderLifecycleMixin](../renderlifecyclemixin#action-setrendererror) | | **attachRenderingBackend**
<B>(backend: B, setup: () => RenderingBackendCallbacks<B>) => void
| attach a GPU/Canvas2D backend and install the upload + render autorun pair. Idempotent: re-calling swaps the backend and does not run `setup` again, so the callbacks and everything they close over are the first call's. | [RenderLifecycleMixin](../renderlifecyclemixin#action-attachrenderingbackend) | | **stopActiveFetch**
() => void | Abort the in-flight fetch (if any) and retire its slot. The shared preamble of both cancel paths; the difference between them is only what they do to `fetchCanceled` / `fetchGeneration` afterward. | [FetchMixin](../fetchmixin#action-stopactivefetch) | | **openStatusStream**
(isCurrent: () => boolean) => StatusStream | Open one operation's slot on the display's status field: an RPC `statusCallback` throttled through the display-wide window and guarded so a callback that fires after the node is torn down (RPCs resolve their status stream asynchronously) is a safe no-op, plus the `clear` that retires the slot when the operation ends.

**Every operation on the display opens one**, and the two come back together because an operation that never retires goes on voting for a phase that is over. The viewport fetch (`runFetch`), the clustering run and a lent `createStopTokenRotation` are three of them on one field; before ADR-081 each blanked the field outright and the last one to finish decided what the other two were still saying.

`isCurrent` is required and has no "node is alive" default, because alive is not the interesting question: a *superseded* fetch is on a live node, and its late status repainting the overlay of the fetch that replaced it is the failure this guards. `runFetch` passes `!isStale()`, which is what every display gets for free through `ctx.statusCallback`; a caller outside a fetch (the clustering autorun) passes its own run's flag. Defaulting to `isAlive` made the loose answer the easy one and five displays took it.

Declared this early only so `runFetch` can put one on every `FetchContext`. | [FetchMixin](../fetchmixin#action-openstatusstream) | | **cancelFetch**
() => void | cancel any in-flight fetch and bump fetchGeneration (always bumps, so callers can retrigger fetch autoruns even when nothing was in flight). This is the *internal* reset `clearAllRpcData` runs — it clears any user-cancel flag so the retrigger actually re-fetches. | [FetchMixin](../fetchmixin#action-cancelfetch) | | **cancelFetchByUser**
() => void | User-initiated cancel from the loading overlay. Stops the in-flight fetch and lands in a durable `fetchCanceled` state. Unlike `cancelFetch`, it does NOT bump fetchGeneration — so the fetch autoruns don't immediately restart the load. The user retries via `reload` (the overlay's retry button), or it clears on the next viewport change. | [FetchMixin](../fetchmixin#action-cancelfetchbyuser) | | **beforeDestroy**
() => void | Release an in-flight fetch's stop token on teardown. Without this, a display destroyed mid-fetch (track/view closed while loading) never signals the worker to abort the now-useless work, and its in-flight HTTP reads keep downloading. MST auto-chains lifecycle hooks, so a composing display can still define its own beforeDestroy. | [FetchMixin](../fetchmixin#action-beforedestroy) | | **endFetch**
(current: boolean, stopToken: StopToken) => void | The `finally` half of `runFetch`'s bookkeeping, an action of its own because `runFetchOnce`'s `finally` resumes on a microtask the flow does not own — a direct volatile write there is outside the action context, which is the one thing hoisting the sequence into a shared function costs. The stale branch is a superseded fetch: whoever superseded it — a newer `begin`, or `cancel` — already released this token. | [FetchMixin](../fetchmixin#action-endfetch) | | **runFetch**
(work: (ctx: FetchContext) => Promise<void>) => Promise<void> | Run a cancel-safe fetch (cancels any prior). The work callback gets a FetchContext with a stopToken to forward to the RPC and an isStale() check to short-circuit commits once the user has moved on.

**The MST-flow wrapper over the shared `runFetchOnce` sequence**, and only the wrapper: the begin/clear/run/commit/error/end order, and the rules that keep a superseded run from writing back, are the same function every other fetch in the tree runs. What this adds is the observable bookkeeping a display needs — `isLoading` through `activeStopToken`, `fetchGeneration`, the user-cancel clear — and the flow itself, which is an action, so `work`'s synchronous prefix runs untracked wherever a fetch autorun calls this. | [FetchMixin](../fetchmixin#action-runfetch) |