Reproduction on Linux with Chromium: right-click README.md in Files and click Copy mention.
Before: the menu closes without writing the clipboard or showing a toast. The file tree's document mousedown handler restores focus before the app menu's click handler runs.
After: Copy mention writes README.md and shows Mention copied. Add to chat also inserts the mention.
The focused browser regression test fails on upstream main with zero clipboard calls and passes with the focus handoff fix.