New hint for clients that grab the keyboard

[Matti Virkkunen <mvirkkunen gmail com>]

Hi,

I'd like to propose a new window state hint for applications that 
currently grab the entire keyboard.

Software such as VNC/RDP clients and virtual machine viewers generally 
want to grab the entire keyboard when focused, so that everything 
(including WM key combinations) can be captured. This however leaves the 
user unable to escape the application using the keyboard, e.g. by 
switching to another window or desktop. It would be nice if the user 
could whitelist some WM key combinations (for example, the key 
combinations for switching virtual desktops) so that they work even if 
the current window wants to grab most keys.

XGrabKeyboard is a bit of a drastic measure because it's very difficult 
to (or impossible to reliably) exclude some keys from the grab, such as 
window manager key combinations. I propose a way to "grab the keyboard" 
without using XGrabKeyboard.

The new window state could be called something like 
_NET_WM_STATE_GRAB_KEYBOARD. What it would do is disable all WM key 
combinations when the window has input focus, except a user-defined 
whitelist. The whitelist could be a part of WM configuration, so that 
the same keys work consistently across different apps. When using this 
window state, clients wouldn't need to use XGrabKeyboard at all. Clients 
can detect the availability of the state via _NET_SUPPORTED and fall 
back to using XGrabKeyboard if it's unavailable.

Rationale for putting this in the WM:

1) Usually the WM is what uses the most (if not all) passive grabs on 
the user's system.
2) The WM knows what hotkeys it uses and offers a configuration 
interface for them, making it the easiest for the WM to manage the list 
of whitelisted hotkeys.
3) Implementation of this in the WM should be fairly straightforward: 
Remove passive grabs when a window in this state receives focus, and 
re-grab the keys when focus is lost.

As a further improvement, a list of hotkeys that work even in grab mode 
could be provided as another hint, so that applications can provide a 
menu for people to input them into the remote/virtual machine. Some way 
of informing the few apps besides the WM that use passive grabs about 
the grab state so that they can release/re-grab their keys could also be 
considered.

Related to the issue of switching away from windows that grab the 
keyboard I found this feature request: 
https://bugs.freedesktop.org/show_bug.cgi?id=4286 , which seems like a 
somewhat better way of implementing this. However the last status update 
says it's possibly scheduled for "XI2.1" which, at least according to 
Google, doesn't even seem to exist yet.


- Matti Virkkunen