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Fix auto-resize destroying horizontal scroll positions #567

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Merged
ochafik merged 2 commits into from
Mar 26, 2026
+17 −12
Merged

Fix auto-resize destroying horizontal scroll positions #567

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e61e1c2
Fix auto-resize destroying horizontal scroll positions
mel-anthropic Mar 25, 2026
0d1e17c
Skip size-changed measurement during fullscreen
mel-anthropic Mar 25, 2026
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29 changes: 17 additions & 12 deletions src/app.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -1287,26 +1287,31 @@ export class App extends Protocol<AppRequest, AppNotification, AppResult> {
scheduled = true;
requestAnimationFrame(() => {
scheduled = false;

// Hosts determine the container size in fullscreen, so size-changed
// notifications are not needed.
if (this._hostContext?.displayMode === "fullscreen") {
return;
}

const html = document.documentElement;

// Measure actual content size by temporarily overriding html sizing.
// Width uses fit-content so content wraps at the host-provided width.
// Measure actual content height by temporarily overriding html sizing.
// Height uses max-content because fit-content would clamp to the viewport
// height when content is taller than the iframe, causing internal scrolling.
const originalWidth = html.style.width;
//
// Width uses window.innerWidth instead of measuring via fit-content.
// Setting html.style.width to fit-content forces a synchronous reflow at
// 0px width for responsive apps (whose content derives width from the
// container rather than having intrinsic width). This causes the browser
// to clamp scrollLeft on any horizontal scroll containers to 0, permanently
// destroying their scroll positions.
const originalHeight = html.style.height;
html.style.width = "fit-content";
html.style.height = "max-content";
const rect = html.getBoundingClientRect();
html.style.width = originalWidth;
const height = Math.ceil(html.getBoundingClientRect().height);
html.style.height = originalHeight;

// Compensate for scrollbar width on Linux/Windows where scrollbars consume space.
// On systems with overlay scrollbars (macOS), this will be 0.
const scrollbarWidth = window.innerWidth - html.clientWidth;

const width = Math.ceil(rect.width + scrollbarWidth);
const height = Math.ceil(rect.height);
const width = Math.ceil(window.innerWidth);

// Only send if size actually changed (prevents feedback loops from style changes)
if (width !== lastWidth || height !== lastHeight) {
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