Fix auto-resize destroying horizontal scroll positions

[mel-anthropic]

Summary

Fix two issues in the SDK's auto-resize measurement that cause visual glitches during viewport transitions (e.g. inline ↔ fullscreen).

Changes

Fix horizontal scroll position destruction

setupSizeChangedNotifications() temporarily sets document.documentElement.style.width = 'fit-content' on every ResizeObserver callback to measure intrinsic content width. For responsive apps that derive their width from the container (width: 100%, flex: 1, etc.), fit-content resolves to 0px.

The getBoundingClientRect() call forces a synchronous reflow at 0px width. During this reflow, the browser clamps scrollLeft on all horizontal scroll containers to 0 (since scrollLeft cannot exceed scrollWidth - clientWidth, which is 0). When the style is restored, scrollLeft stays clamped — the browser does not un-clamp it.

This manifests as horizontal scroll views losing their position whenever the viewport resizes, which is most visible during inline ↔ fullscreen transitions.

Fix: Replace the fit-content width measurement with window.innerWidth. The height measurement (max-content) is preserved since hosts rely on it. Neither the iOS nor web host uses the width value from size-changed notifications — both only consume height — so this change has no functional impact on hosts.

Skip size-changed measurement during fullscreen

Hosts determine the container size in fullscreen mode, so size-changed notifications are not needed. The temporary max-content height override used for measurement causes visible vertical jitter during animated fullscreen transitions by briefly reflowing the document on every animation frame.

Fix: Skip measurement entirely when hostContext.displayMode === "fullscreen". Verified that all three hosts (iOS, Android, Web) ignore the size-changed height value for layout during fullscreen.

Testing

• Confirmed scroll position fix on iOS: scrollLeft dropped from 6406 to 772 when entering fullscreen before the fix; stays stable after
• Applied equivalent CSS fix in Safari Web Inspector and verified scroll position preserved
• Confirmed fullscreen skip eliminates vertical jitter during animated transitions
• All existing tests pass (88/88)

[pkg-pr-new]

Open in StackBlitz

@modelcontextprotocol/ext-apps

npm i https://pkg.pr.new/@modelcontextprotocol/ext-apps@567

@modelcontextprotocol/server-basic-preact

npm i https://pkg.pr.new/@modelcontextprotocol/server-basic-preact@567

@modelcontextprotocol/server-basic-react

npm i https://pkg.pr.new/@modelcontextprotocol/server-basic-react@567

@modelcontextprotocol/server-basic-solid

npm i https://pkg.pr.new/@modelcontextprotocol/server-basic-solid@567

@modelcontextprotocol/server-basic-svelte

npm i https://pkg.pr.new/@modelcontextprotocol/server-basic-svelte@567

@modelcontextprotocol/server-basic-vanillajs

npm i https://pkg.pr.new/@modelcontextprotocol/server-basic-vanillajs@567

@modelcontextprotocol/server-basic-vue

npm i https://pkg.pr.new/@modelcontextprotocol/server-basic-vue@567

@modelcontextprotocol/server-budget-allocator

npm i https://pkg.pr.new/@modelcontextprotocol/server-budget-allocator@567

@modelcontextprotocol/server-cohort-heatmap

npm i https://pkg.pr.new/@modelcontextprotocol/server-cohort-heatmap@567

@modelcontextprotocol/server-customer-segmentation

npm i https://pkg.pr.new/@modelcontextprotocol/server-customer-segmentation@567

@modelcontextprotocol/server-debug

npm i https://pkg.pr.new/@modelcontextprotocol/server-debug@567

@modelcontextprotocol/server-map

npm i https://pkg.pr.new/@modelcontextprotocol/server-map@567

@modelcontextprotocol/server-pdf

npm i https://pkg.pr.new/@modelcontextprotocol/server-pdf@567

@modelcontextprotocol/server-scenario-modeler

npm i https://pkg.pr.new/@modelcontextprotocol/server-scenario-modeler@567

@modelcontextprotocol/server-shadertoy

npm i https://pkg.pr.new/@modelcontextprotocol/server-shadertoy@567

@modelcontextprotocol/server-sheet-music

npm i https://pkg.pr.new/@modelcontextprotocol/server-sheet-music@567

@modelcontextprotocol/server-system-monitor

npm i https://pkg.pr.new/@modelcontextprotocol/server-system-monitor@567

@modelcontextprotocol/server-threejs

npm i https://pkg.pr.new/@modelcontextprotocol/server-threejs@567

@modelcontextprotocol/server-transcript

npm i https://pkg.pr.new/@modelcontextprotocol/server-transcript@567

@modelcontextprotocol/server-video-resource

npm i https://pkg.pr.new/@modelcontextprotocol/server-video-resource@567

@modelcontextprotocol/server-wiki-explorer

npm i https://pkg.pr.new/@modelcontextprotocol/server-wiki-explorer@567

commit: 0d1e17c