How to reproduce:
1. Create and open a new Universal 2D project. It seems to be a little easier to reproduce with this project but I suspect it's not limited to it.
2. Open the Inspector and make it a floating window
3. Select the “Main Camera“ GameObject
4. Press the lock symbol to lock the Inspector window
5. Focus the main Editor window
6. Try selecting the “Layer“ dropdown menu in the Inspector window. This can be reproduced with any dropdown. However it seems to be easier to reproduce if no highlight from hover over a control occurs before you click on the dropdown.

Expected result: The dropdown menu opens immediately and displays its options
Actual result: The dropdown menu requires two clicks before the options appear

Reproducible with: 6000.3.0a2, 6000.3.1f1, 6000.4.0b1, 6000.5.0a2
Not reproducible with: 6000.0.64f1, 6000.3.0a1

Reproducible on: Ubuntu 22.04 (only reproducible with the X11 windowing system). Ubuntu 24.04(Wayland only).
Not reproducible on: macOS 26.1, Windows 11

Note: This issue is not exclusive to the Inspector window