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blocksuite/docs/api/@blocksuite/std/gfx/variables/viewportRuntimeConfig.md

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+[**BlockSuite API Documentation**](../../../../README.md)
+
+***
+
+[BlockSuite API Documentation](../../../../README.md) / [@blocksuite/std](../../README.md) / [gfx](../README.md) / viewportRuntimeConfig
+
+# Variable: viewportRuntimeConfig
+
+> `const` **viewportRuntimeConfig**: `object`
+
+Process-wide defaults applied to every Viewport at construction.
+
+Platforms that need different behavior (e.g. mobile/iOS, which must clamp the
+zoom floor and defer DOM mutations during gestures to avoid WKWebView process
+termination) override these once at startup, before any editor mounts. This
+guarantees both the editor and the readonly preview viewports are born with
+the same limits — avoiding the race and wrong-instance problems of patching a
+single Viewport asynchronously after it has already mounted.
+
+Desktop leaves these untouched, so its behavior is unchanged.
+
+## Type Declaration
+
+### CANVAS\_DPR\_CAP\_BY\_ZOOM
+
+> **CANVAS\_DPR\_CAP\_BY\_ZOOM**: \[`number`, `number`\][]
+
+Caps the canvas backing-store device-pixel-ratio at low zoom.
+
+Each entry is `[zoomThreshold, dprCap]`, sorted ascending by threshold.
+When the live zoom is below a threshold, the corresponding cap bounds the
+effective dpr used to size canvases. Far-out zoom makes content tiny on
+screen, so a full retina backing store is wasted memory — on iOS that waste
+is what pushes WKWebView past its compositing budget and crashes the web
+content process during pan/zoom.
+
+Empty (the desktop default) means no cap: canvases always use the raw
+`window.devicePixelRatio`, so desktop behavior is unchanged.
+
+### LOW\_ZOOM\_GESTURE\_ACTIVE\_BLOCK\_LIMIT
+
+> **LOW\_ZOOM\_GESTURE\_ACTIVE\_BLOCK\_LIMIT**: `number` = `0`
+
+During low-zoom gesture survival mode, keep only a tiny subset of DOM blocks
+as real active DOM (selected + a few nearby blocks). `0` keeps the legacy
+behavior where every viewport block remains visually mounted as `survival`.
+
+### LOW\_ZOOM\_GESTURE\_ACTIVE\_DISTANCE\_RATIO
+
+> **LOW\_ZOOM\_GESTURE\_ACTIVE\_DISTANCE\_RATIO**: `number` = `0.35`
+
+Distance threshold (as a fraction of the viewport's shorter side) used to
+decide whether an unselected viewport block counts as "nearby" to the
+current selection during low-zoom gesture survival mode.
+
+### OVERSCAN\_RATIO
+
+> **OVERSCAN\_RATIO**: `number` = `0`
+
+Fraction by which the *render/activation* viewport bound is enlarged on
+every side (see Viewport.overscanViewportBounds). Pre-painting a
+margin around the visible area means moderate pan/zoom gestures move into
+content that is already mounted and rasterized, so it does not blank out
+and wait for the post-gesture refresh.
+
+Memory grows by roughly `(1 + 2 * ratio) ** 2`, so this must stay modest
+and be paired with a zoom floor + dpr cap on mobile. `0` (desktop default)
+makes Viewport.overscanViewportBounds identical to
+Viewport.viewportBounds, leaving desktop behavior unchanged.
+
+This governs the *canvas* render bound only (see
+Viewport.overscanViewportBounds). It enlarges the canvas backing
+stores, so memory grows with the overscan area. Keep it modest and pair it
+with the mobile zoom floor + dpr cap so connectors/elements stay painted
+through a gesture without pushing WKWebView over budget.
+
+### OVERSCAN\_RATIO\_BLOCK
+
+> **OVERSCAN\_RATIO\_BLOCK**: `number` = `0`
+
+Like [OVERSCAN\_RATIO](#overscan_ratio) but for the *DOM block mounting* bound (see
+Viewport.overscanBlockBounds). This one is expensive: every
+mounted block becomes its own composited layer subtree in the WebContent
+process, so enlarging it multiplies resident memory and is what pushes the
+process toward an iOS jetsam kill. Keep this small (or `0`) even when
+[OVERSCAN\_RATIO](#overscan_ratio) is generous. `0` (desktop default) leaves block
+mounting on the exact visible bound, unchanged from upstream.
+
+### POST\_GESTURE\_REFRESH\_DELAY
+
+> **POST\_GESTURE\_REFRESH\_DELAY**: `number` = `800`
+
+Delay (ms) before the post-gesture refresh repaints canvases and reactivates
+blocks, used only when [SKIP\_REFRESH\_DURING\_GESTURE](#skip_refresh_during_gesture) is true. The same
+value drives both the canvas and block refresh timers so they fire together
+(avoiding the "blocks appear, then connectors" staggered reveal). Desktop
+never enters that code path, so this is mobile-only.
+
+### SKIP\_REFRESH\_DURING\_GESTURE
+
+> **SKIP\_REFRESH\_DURING\_GESTURE**: `boolean` = `false`
+
+### VIEWPORT\_REFRESH\_MAX\_INTERVAL
+
+> **VIEWPORT\_REFRESH\_MAX\_INTERVAL**: `number` = `120`
+
+### VIEWPORT\_REFRESH\_PIXEL\_THRESHOLD
+
+> **VIEWPORT\_REFRESH\_PIXEL\_THRESHOLD**: `number` = `18`
+
+### ZOOM\_MAX
+
+> **ZOOM\_MAX**: `number`
+
+### ZOOM\_MIN
+
+> **ZOOM\_MIN**: `number`