[Qt-interest] QWidget::grabKeyboard() and window managers

[Pedro Lopez-Cabanillas]

Hi,

I've made an application based on Qt4, a virtual MIDI piano keyboard 
(http://vmpk.sourceforge.net) allowing the users to press the alphanumeric 
computer keys to play MIDI notes on MIDI synths. There is a configurable  
option "Grab Keyboard" wanted by most users. I've implemented it using  
QWidget::grabKeyboard(). Enabling this option, the piano widget should 
receive all key events even when it is not focused. This allows for instance 
use the mouse on other widgets like the volume knob without having to click 
on the piano again to give it back the focus.

This feature works in Windows and Mac OSX limited to the current application, 
so the keyboard is not globally grabbed. It works very well (with global 
grab) in Linux using KDE3/4 desktops with the standard kwin window manager. 
It also works with Enlightenment and Window Maker, but fails in Metacity, 
Compiz and some other window managers. In other words: it is broken for most 
Gnome users. Using this option also prevents normal usage of the drop down 
menus on GTK2 applications. 

I've seen that QWidget::grabKeyboard() is implemented in Qt/X11 using 
XGrabKeyboard(3), a public and documented X11 API function. Do you know why 
those WMs and toolkit don't support it? Is there any alternative? Can you 
give me some advice? 

Regards,
Pedro