CSS isolation property may affect contenteditable stacking context

Phenomenon

When a contenteditable element has the CSS isolation: isolate property, it creates a new stacking context. This may affect how selection handles and IME candidate windows are positioned relative to the contenteditable.

Reproduction example

1. Create a contenteditable div with isolation: isolate.
2. Use an IME to compose text.
3. Observe the position of the IME candidate window.
4. Try to select text and observe selection handles.
5. Compare with a contenteditable without isolation.

Observed behavior

• In Safari on macOS, isolation may affect IME candidate window positioning.
• Selection handles may be positioned incorrectly.
• Z-index stacking may be affected.
• Overlays may not appear correctly.

Expected behavior

• CSS isolation should not affect contenteditable UI elements.
• IME candidate windows should be positioned correctly.
• Selection handles should appear in the right place.