Proposing _NET_WM_STATE_FULLSCREEN_EXCLUSIVE

["Ryan C. Gordon" <icculus icculus org>]

I've been doing Linux video games for over a decade now, and we've never 
really had a satisfactory means to handle fullscreen games on X11. I see 
this mailing list discussed fullscreen problems briefly in February, but 
I wanted to make a formal proposal of how the system should handle 
applications that want to "go fullscreen."

This is a first draft that Sam Lantinga and I put together, envisioning 
how we would like SDL to cooperate with the Window Manager to solve the 
problem.

Being a first draft, anything that's silly is totally open to change. We 
have an SDL patch that implements the client side of this spec, but no 
one's tried to implement the Window Manager side of things yet.

Comments and criticism are appreciated!

Thanks,
--ryan.



This is an addition to the spec for _NET_WM_STATE:
     http://standards.freedesktop.org/wm-spec/1.3/ar01s05.html#id3076525

Rationale:

There are some problems with the current _NET_WM_STATE_FULLSCREEN hint 
that we would like to address.

First, it is not explicit in its instructions to Window Managers; for 
example, we found some Window Managers required the window to be marked 
resizable (Metacity) and others did not (XFCE); it's not clear which is 
correct. It doesn't address virtual desktop size verses physical 
resolution. This proposal tries to be very clear about exactly what 
steps are required.

Second, the window manager needs to be responsible for changing 
resolutions. If an app tries to use XVidMode, the virtual desktop will 
not be resized and _NET_WM_STATE_FULLSCREEN will size the window 
incorrectly. If an app instead tries to use XRandR, the rest of the 
desktop may change irreversibly; icons may move around, other app 
windows may shrink down. Worse still, if an app changes the resolution 
with XVidMode or XRandR and then crashes before cleaning itself up, the 
desktop will remain in the wrong state until the user takes heroic 
measures to fix it. Moving this operation into the Window Manager and 
associating it with a specific window lets the system know that this is 
a temporary state change, instead of two discrete actions that may be 
unrelated, so that it can protect desktop state appropriately. This also 
has the side benefit of centralizing a complex negotiation with X11 
extensions and multiple processes into one place--the Window 
Manager--instead of every application.

Certainly _NET_WM_STATE_FULLSCREEN still has a place; it still makes 
good sense that the example given, a presentation program, use that 
hint. For applications like video games, which need more control over 
window geometry and specific screen resolutions, a new hint is extremely 
useful.


Specification:

A Window Manager supporting this specification MUST specify 
_NET_WM_STATE_FULLSCREEN_EXCLUSIVE in the list of atoms reported by the 
_NET_SUPPORTED property.

(Added to the list of _NET_WM_STATE hints...)
_NET_WM_STATE_FULLSCREEN_EXCLUSIVE indicates that the Window Manager 
MUST change the resolution of the window's screen to one that most 
closely matches the window's current dimensions. If no available 
resolution matches exactly, the Window Manager MUST select the closest 
available resolution larger than the window. The Window Manager MUST 
center the window within the new resolution, remove any window 
decorations, retain its original geometry, and grant that window input 
focus. If the chosen resolution does not match the window geometry, the 
Window Manager MUST obscure the rest of the screen so that the window is 
the only thing visible. If there is no resolution that can completely 
contain the window's undecorated geometry, the Window Manager MUST 
refuse to allow this hint.

If the window loses input focus while fullscreen, the Window Manager 
MUST revert the resolution change and iconify the window until it 
regains input focus. The Window Manager MUST protect desktop state (icon 
positions, geometry of other windows, etc) during resolution change, so 
that the state will be unchanged when the window ceases to be marked as 
fullscreen. The _NET_WM_STATE_FULLSCREEN_EXCLUSIVE and 
_NET_WM_STATE_FULLSCREEN hints MUST NOT be used together by an 
application; in such a case, the Window Manager must reject the 
_NET_WM_STATE_FULLSCREEN hint.

If more than one window on the same screen requests the 
_NET_WM_STATE_FULLSCREEN_EXCLUSIVE hint, the latest window MUST obtain 
its requested resolution and input focus, and all other windows with 
this hint MUST be iconified.

For example, a video game would use this hint.