Please describe the problem:
It is possible to set combination like "Ctrl+Shift" the keyboard layout switcher privatizes it as the shortcut for switching layouts, and disallows all other programs to use it as modifier.

Steps to reproduce:
1. Open "Keyboard preferences" in Keyboard layout indicator applet.
2. Go to Layouts.
3. Add some keyboard layouts.
4. Click "Other Options".
5. Under "Layout switching", check "Ctrl+Shift" change layout.
6. Open Firefox.
7. Open several tabs with Ctrl+T.
8. Switch forward through them with multiple "Ctrl+Tab". Notice that everything is ok.
9. Try to switch backward through them with multiple "Ctrl+Shift+Tab"...


Actual results:
Exactly at moment when you press down Ctrl and Shift, keyboard layout changes; after you press Tab, without letting Ctrl and Shift off, Firefox switches one tab forward instead of one tab backward.

Expected results:
Firefox should go one tab back.
Layout should not change.

Overall, the layout should change not at the time when you press down Ctrl and Shift, but only at the time when you let go one of them, and only if you didn't use this combination as modifier for another key.

Does this happen every time?
Yes.

Other information:
In windows, everything is working exactly as desired - layouts are changed only at the time when you let go the combination.