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:
0. Login to GNOME (the steps below are for 2.24, but should be similar for other versions).
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.

This bug was filed against GNOME, and they told it is X.Org problem: http://bugzilla.gnome.org/show_bug.cgi?id=558738