Remove will-change: width and use actual element height in PageLayout drag optimizations

[Copilot]

Removes ineffective will-change: width optimization and fixes layout shift from hardcoded size estimation in PageLayout drag operations.

Changes

paneUtils.ts:

• Removed will-change: width from drag styles—provides no benefit for layout properties, only increases memory
• Changed containIntrinsicSize from hardcoded 500px to actual element.offsetHeight to prevent layout shift with content-visibility: auto

PageLayout.test.tsx:

• Updated test to verify will-change is no longer applied during drag operations

Changelog

Changed

• PageLayout.Pane: Removed will-change: width during resize/drag operations
• PageLayout.Pane: containIntrinsicSize now uses actual element height instead of hardcoded 500px

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

Test resizable panes in PageLayout stories—drag behavior should be unchanged, but more efficient.

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

Summary

Two small fixes to paneUtils.ts drag optimizations:

1. Remove will-change: width - This property only benefits compositor-friendly properties like transform and opacity. For layout-triggering properties like width, it provides no benefit and may increase memory usage.

2. Use actual element height for containIntrinsicSize - Instead of hardcoded 500px, use element.offsetHeight for accurate size estimation. This prevents layout shift when content-visibility: auto is applied.

File to modify

packages/react/src/PageLayout/paneUtils.ts

Changes

export function setContainmentOptimizations(element: HTMLElement | null) {
  if (!element) return
  element.style.contain = 'layout style paint'
  element.style.contentVisibility = 'auto'
  element.style.containIntrinsicSize = `auto ${element.offsetHeight}px`  // Use actual height
  element.style.pointerEvents = 'none'
}

export function setDraggingStyles({handle, pane, content}: DraggingStylesParams) {
  handle?.style.setProperty('background-color', 'var(--bgColor-accent-emphasis)')
  handle?.style.setProperty('--draggable-handle--drag-opacity', '1')
  handle?.style.setProperty('--draggable-handle--transition', 'none')
  // Remove: pane?.style.setProperty('will-change', 'width')
  setContainmentOptimizations(content)
  setContainmentOptimizations(pane)
}

export function removeDraggingStyles({handle, pane, content}: DraggingStylesParams) {
  handle?.style.removeProperty('background-color')
  handle?.style.removeProperty('--draggable-handle--drag-opacity')
  handle?.style.removeProperty('--draggable-handle--transition')
  // Remove: pane?.style.removeProperty('will-change')
  removeContainmentOptimizations(content)
  removeContainmentOptimizations(pane)
}

Rationale

Change	Reason
Remove will-change: width	Only helps transform/opacity, not layout properties. May hurt performance.
Use element.offsetHeight	Prevents layout shift from incorrect size guess. Single read at drag start is acceptable cost.
Keep content-visibility on both	No harm if content is on-screen; helps if content is scrolled/off-screen.

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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.