Steps to reproduce:
1. Run attached program. (The node orientation of the Scene is RTL, and the letters "A", "B" and "C" are the mnemonics of the menus.)
2. Press "Alt", there should be a line displayed under "A", "B" and "C".

But the line is place under other letters.

This happens from fx 8, not a regression.
If the text of the menu is RTL or BiDi, the underline is correct. The problem happens only when the text is LTR but the node orientation is RTL.