Preserve element size during drag with contain-intrinsic-size

[Copilot]

When dragging resizable panes, content-visibility: auto causes off-screen content to collapse, triggering layout shifts and scroll jumping. This maintains the performance optimization while stabilizing layout.

Implementation

paneUtils.ts - Modified drag style management:

• setDraggingStyles(): Captures element dimensions via getBoundingClientRect() and sets contain-intrinsic-size inline style with auto keyword before applying data-dragging attribute
• removeDraggingStyles(): Removes contain-intrinsic-size after drag ends

CSS - No changes needed, content-visibility: auto remains active during drag for performance

Mechanism

// Before setting data-dragging attribute (which triggers content-visibility: auto)
if (pane) {
  const rect = pane.getBoundingClientRect()
  // Use 'auto' to track actual size as width changes during drag
  pane.style.setProperty('contain-intrinsic-size', `auto ${rect.width}px auto ${rect.height}px`)
}
pane?.setAttribute('data-dragging', 'true')

The contain-intrinsic-size property with auto keyword provides a size hint to the browser when content-visibility: auto skips rendering. The auto allows the browser to track the actual rendered size as the pane width changes dynamically during drag operations via --pane-width CSS variable updates.

Changelog

Changed

• setDraggingStyles() now captures and preserves element dimensions during drag operations using auto keyword for dynamic sizing
• removeDraggingStyles() cleans up dimension hints after drag

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Tests verify contain-intrinsic-size is applied with auto <width>px auto <height>px format during both pointer and keyboard drag operations, and removed on drag end. Verify dragging near scrollable container edges shows no layout jumps.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

Original prompt

Overview

This is an alternative approach to PR #7445 that maintains the content-visibility: auto optimization while preventing size changes during drag operations.

Problem

When dragging elements, the content-visibility: auto CSS property causes the browser to skip rendering the content of off-screen elements. This results in the element's size changing during drag operations, which causes scroll jumping and layout instability.

PR #7445 simply removes content-visibility: auto from dragging elements, which loses the performance optimization.

Proposed Solution

Instead of removing content-visibility: auto, we should:

1. At drag start: Capture the element's current dimensions (width and height)
2. Set contain-intrinsic-size: Write the captured dimensions as an inline style (contain-intrinsic-size: <width>px <height>px) on the element before the data-dragging='true' attribute is applied (which triggers content-visibility: auto via CSS)
3. At drag end: Remove the inline contain-intrinsic-size style when dragging ends

This approach:

• Keeps the content-visibility: auto optimization for performance during drag
• Maintains the element's initial size via contain-intrinsic-size, preventing layout shifts and scroll jumping
• Is fully reversible when drag ends

Implementation Details

Files to modify:

1. packages/react/src/PageLayout/paneUtils.ts:

   • In setDraggingStyles(): Before setting data-dragging attribute, capture the current getBoundingClientRect() dimensions of both pane and contentWrapper, then set contain-intrinsic-size as an inline style
   • In removeDraggingStyles(): Remove the contain-intrinsic-size inline style

2. packages/react/src/PageLayout/PageLayout.module.css:

   • Keep the content-visibility: auto rule in the .Pane[data-dragging='true'], .ContentWrapper[data-dragging='true'] selector (do NOT remove it like PR Remove content-visibility from dragging elements #7445 does)

Example implementation for setDraggingStyles:

export function setDraggingStyles({handle, pane, contentWrapper}: DraggingStylesParams) {
  // Handle visual feedback
  handle?.style.setProperty('--draggable-handle--bg-color', 'var(--bgColor-accent-emphasis)')
  handle?.style.setProperty('--draggable-handle--drag-opacity', '1')
  handle?.style.setProperty('--draggable-handle--transition', 'none')

  // Capture current dimensions and set contain-intrinsic-size BEFORE setting data-dragging
  // This ensures the element maintains its size when content-visibility: auto is applied
  if (pane) {
    const rect = pane.getBoundingClientRect()
    pane.style.setProperty('contain-intrinsic-size', `${rect.width}px ${rect.height}px`)
  }
  if (contentWrapper) {
    const rect = contentWrapper.getBoundingClientRect()
    contentWrapper.style.setProperty('contain-intrinsic-size', `${rect.width}px ${rect.height}px`)
  }

  // Set attribute for CSS containment (O(1) direct selector, not descendant)
  pane?.setAttribute(DATA_DRAGGING_ATTR, 'true')
  contentWrapper?.setAttribute(DATA_DRAGGING_ATTR, 'true')
}

Example implementation for removeDraggingStyles:

export function removeDraggingStyles({handle, pane, contentWrapper}: DraggingStylesParams) {
  handle?.style.removeProperty('--draggable-handle--bg-color')
  handle?.style.removeProperty('--draggable-handle--drag-opacity')
  handle?.style.removeProperty('--draggable-handle--transition')

  pane?.removeAttribute(DATA_DRAGGING_ATTR)
  contentWrapper?.removeAttribute(DATA_DRAGGING_ATTR)
  
  // Remove contain-intrinsic-size after removing data-dragging
  pane?.style.removeProperty('contain-intrinsic-size')
  contentWrapper?.style.removeProperty('contain-intrinsic-size')
}

Testing

1. Drag an element that has content-visibility: auto applied
2. Verify that the element maintains its size during the drag (no collapse/expansion)
3. Confirm that no scroll jumping occurs when dragging near the edges of a scrollable container
4. Verify the contain-intrinsic-size inline style is present during drag and removed after

Related

• Closes the issue that PR Remove content-visibility from dragging elements #7445 was addressing
• Alternative to PR Remove content-visibility from dragging elements #7445

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Or, apply the integration-tests: skipped manually label to skip these checks.

[mattcosta7]

@copilot should we use auto width here instead of a fixed width, since we're changing width or is this ok

[Copilot]

Good catch! Updated to use auto ${width}px auto ${height}px format. This allows the browser to track the actual rendered size rather than using a fixed size captured at drag start, which is important since the pane width changes dynamically during drag via --pane-width CSS variable updates.

Fixed in commit 1d93cdb.

[github-actions]

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