Steps to reproduce:
1) Go to 'View' menu and make sure the 'Main Menu bar' is turned off.
2) Go to 'Tools > Options > Library > Appearance' and turn the 'always show the tab bar' option ON
OR
Open a second tab (for example, by pressing Ctrl+T or middle-clicking on a node)
3) In titlebar, click on 'Menu', so it expand to the right.
4) Move the mouse pointer away from menu.
A magenta rectangle appears at the right side of titlebar, when menu is collapsed again.
Hovering the tab bar or clicking outside MM window makes the rectangle go away.

Other details:
- Tested in Windows 7 with Aero
- Reproduced with Pulse and Vitreous skins
- Reproduced with different languages
Can anyone try to reproduce this, before I create a new ticket in Mantis?