Recently we received a bug report about input method switching.
Sometimes the input method switch is hard to trigger if the user press the shortcut key very quickly.

I just find a key press sequence to re-produce this bug very easily:
1. Configure the switch-input-source short cut key to Ctrl+Space;
2. Press Ctrl, Press Space, Release Ctrl, Release Space;

With the above sequence, the input method switch can't be trigger in gnome-shell,
 but XFCE is okay.
I guess in XFCE the input method switch is handled by ibus-ui-gtk3,
in gnome-shell 3.6, this is handled by gnome-settings-daemon.

In ibus panel, I think its logic is like follow:
1. detect the shortcut key when the key is pressed;
2. when the key press event is received, check whether the modifier key is still in pressed state;
3. if so, the shortcut key is activated;

The original bug report:
https://bugzilla.redhat.com/show_bug.cgi?id=924046