Re: New hint for clients that grab the keyboard

On Mon, Sep 27, 2010 at 1:16 PM, Mark Tiefenbruck <[email protected]> wrote:
> I don't have any thoughts on a standard, but I'll mention how Fluxbox
> handles this for inspiration. The user can precede keyboard shortcuts
> in the config file by a namespace, and he can create commands that
> switch the set of keyboard shortcuts that are currently in effect.
> I've been using this for years to disable all but a few shortcuts when
> using Xnest or Xephyr. Our users also use it to enter a window moving
> or resizing mode, for example, so they can reuse simple keystrokes
> depending on the task they want to perform. I guess the key addition
> you want to make here is that the window manager should be able to
> change namespaces when a particular window is focused (users who have
> read our source could do this, as there are event hooks to run
> commands when the focus changes; we don't advertise this feature
> because of all the stupid things you could do with it), and for
> applications that grab the keyboard to let the window manager handle
> it (maybe we could add something to _NET_WM_SUPPORTED).

I'd like to suggest a root window property to pair with this, which is
set by the Window Manager, similar to _NET_ACTIVE_WINDOW.

Call the property _NET_ACTIVE_WINDOW_EXCLUSIVE_KEYBOARD, it has type WINDOW.

When a window is focused and has _NET_WM_STATE_GRAB_KEYBOARD set, then
the window manager SHOULD give the application exclusive keyboard
access.  This means to:
a) remove all passive KeyGrabs on the root window, except for a
minimal set to allow removing focus from the selected window via
keyboard.
The minimal set of KeyGrabs may only include:
 - one or more key bindings which the user would use to move focus off
the window.
 - one or more key bindings which the user would use to close the window.
 - any key bindings on "multimedia keys" (a more precise definition of
these is needed)
 - any key bindings that include a modifier other than Shift,
ScrollLock, NumLock, CapsLock, Alt, and Control
 - any other key binding that the user has explicitly stated should be kept
b) set the active window's id in the root window property
_NET_ACTIVE_WINDOW_EXCLUSIVE_KEYBOARD

The value of _NET_ACTIVE_WINDOW must always be equal to
_NET_ACTIVE_WINDOW_EXCLUSIVE_KEYBOARD at the time
_NET_ACTIVE_WINDOW_EXCLUSIVE_KEYBOARD is set.

When focus leaves a window which has exclusive keyboard access, or the
_NET_WM_STATE_GRAB_KEYBOARD state is removed from the active window,
the window manager MUST take away its exclusive keyboard access.  To
do this, the window manager will remove the
_NET_ACTIVE_WINDOW_EXCLUSIVE_KEYBOARD property from the root window.

Applications other than the window manager may hold passive KeyGrabs
on the root window.  To be compliant, any application which holds a
passive KeyGrab on the root window, at any time, must listen for the
_NET_ACTIVE_WINDOW_EXCLUSIVE_KEYBOARD property on the root window.
When the property is set to a value equal to _NET_ACTIVE_WINDOW, the
application MUST remove any passive KeyGrabs which do not fall in a
minimal set of bindings:
 - any key bindings on "multimedia keys" (a more precise definition of
these is needed)
 - any key bindings that include a modifier other than Shift,
ScrollLock, NumLock, CapsLock, Alt, and Control
 - any other key binding that the user has explicitly stated should be kept



What say you all?

- Dana


>  Mark
>
> On Mon, Sep 27, 2010 at 9:32 AM, Giles Atkinson
> <[email protected]> wrote:
>> Matti,
>>
>> An interesting proposal - I would certainly use it.  As a window manager feature, I guess it would also only work for those key/modifier combinations grabbed by the WM.  It's not clear to me how this would handle applets that grab keys for their own use.  One example would be volume controls, although an application like RDP/VNC would probably not want to grab those, it might want to grab the keys of say a workspace pager implemented outside the WM.
>>
>> Giles
>>
>> -----Original Message-----
>> From: [email protected] [mailto:[email protected]] On Behalf Of Matti Virkkunen
>> Sent: 27 September 2010 00:38
>> To: [email protected]
>> Subject: New hint for clients that grab the keyboard
>>
>> 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
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