Fix auto-resize destroying horizontal scroll positions#567
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ochafik merged 2 commits intomodelcontextprotocol:mainfrom Mar 26, 2026
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The setupSizeChangedNotifications method temporarily sets html.style.width to 'fit-content' to measure intrinsic content width. For responsive apps that derive width from their container (width: 100%, flex: 1, etc.), fit-content resolves to 0px. The synchronous reflow at 0px causes browsers to clamp scrollLeft on all horizontal scroll containers to 0, permanently destroying their scroll positions. 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, so this change has no functional impact on hosts while fixing scroll position clamping for all MCP apps with horizontal scroll views.
@modelcontextprotocol/ext-apps
@modelcontextprotocol/server-basic-preact
@modelcontextprotocol/server-basic-react
@modelcontextprotocol/server-basic-solid
@modelcontextprotocol/server-basic-svelte
@modelcontextprotocol/server-basic-vanillajs
@modelcontextprotocol/server-basic-vue
@modelcontextprotocol/server-budget-allocator
@modelcontextprotocol/server-cohort-heatmap
@modelcontextprotocol/server-customer-segmentation
@modelcontextprotocol/server-debug
@modelcontextprotocol/server-map
@modelcontextprotocol/server-pdf
@modelcontextprotocol/server-scenario-modeler
@modelcontextprotocol/server-shadertoy
@modelcontextprotocol/server-sheet-music
@modelcontextprotocol/server-system-monitor
@modelcontextprotocol/server-threejs
@modelcontextprotocol/server-transcript
@modelcontextprotocol/server-video-resource
@modelcontextprotocol/server-wiki-explorer
commit: |
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 frame.
mel-anthropic
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Mar 26, 2026
Reverts the fullscreen displayMode guard added in modelcontextprotocol#567. Skipping size-changed measurement during fullscreen caused apps to not receive an initial size report after the transition, breaking layout for some apps.
ochafik
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Mar 26, 2026
Reverts the fullscreen displayMode guard added in #567. Skipping size-changed measurement during fullscreen caused apps to not receive an initial size report after the transition, breaking layout for some apps.
ochafik
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Mar 27, 2026
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Summary
Fix a bug where the SDK's auto-resize measurement permanently destroys horizontal scroll positions in MCP apps during viewport transitions (e.g. inline ↔ fullscreen).
Problem
setupSizeChangedNotifications()temporarily setsdocument.documentElement.style.width = 'fit-content'on everyResizeObservercallback to measure intrinsic content width. For responsive apps that derive their width from the container (width: 100%,flex: 1, etc.),fit-contentresolves to 0px.The
getBoundingClientRect()call forces a synchronous reflow at 0px width. During this reflow, the browser clampsscrollLefton all horizontal scroll containers to 0 (sincescrollLeftcannot exceedscrollWidth - clientWidth, which is 0). When the style is restored,scrollLeftstays 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-contentwidth measurement withwindow.innerWidth. The height measurement (max-content) is preserved since hosts rely on it. Neither the iOS nor web host uses thewidthvalue fromsize-changednotifications — both only consumeheight— so this change has no functional impact on hosts.Testing
scrollLeftdropped from 6406 to 772 when entering fullscreen before the fix; stays stable after