CSS contain property may affect contenteditable selection

Phenomenon

When a contenteditable element or its parent has the CSS contain property, selection behavior may be affected. Selection may not extend beyond the contained element, and caret movement may be restricted.

Reproduction example

1. Create a contenteditable div with contain: layout style paint.
2. Try to select text that spans beyond the element boundaries.
3. Try to move the caret outside the element.
4. Observe selection and caret behavior.

Observed behavior

• In Chrome on Windows, CSS contain may restrict selection.
• Selection may not extend beyond contained boundaries.
• Caret movement may be limited.
• Selection ranges may be invalidated.

Expected behavior

• CSS contain should not affect contenteditable selection.
• Or, the behavior should be clearly documented.
• Selection should work normally within contained elements.