Repros in 5.16, 5.17 & 5.18 on desktop app as well as webapp
Steps:
1 - Click the ☰ menu and click the separator line between any two menu items
Expected: The menu doesn't close allowing you to realize you accidentally missed the menu item you were aiming for
Observed: The menu closes without any indication that you miss-clicked
Note: Observed the same behavior on the … ellipsis menu in a message