Caret position jumps unexpectedly after pasting content

Phenomenon

After pasting content into a contenteditable region, the caret position does not end up at the expected location, sometimes jumping to the beginning of the pasted content or to an unexpected position.

Reproduction example

1. Create a contenteditable div with some existing text.
2. Place the caret in the middle of the text.
3. Paste content (Ctrl+V or Cmd+V).
4. Observe the final caret position.

Observed behavior

• In Firefox on Windows, the caret sometimes ends up at the start of the pasted content instead of after it.
• The selection range after paste is inconsistent.

Expected behavior

• The caret should be positioned immediately after the pasted content.