[Xorg] Composite and Background == None windows

[Keith Packard]

I'm having trouble getting Background == None windows to work
right with Composite.  The problem is that when these windows
are mapped, you get garbage contents for the backing pixmap.

Background == None windows are for two reasons:

	1)	To avoid flashing a background pixel which will
		immediately be replaced by "real" contents.

	2)	To capture an image of the screen inside the window

I submit that case 1) is a more "legitimate" usage as 2) breaks in the
presense of different visuals in any case.  So, it's important to
get case 1) right while still letting case 2) do something 
reasonable.

For case 1) (used for menus and window manager frames), it seems 
like the right solution is to avoid painting the window at all 
until the application has managed to fill it entirely with the
correct content.  That would require Damage to be modified to let the
application know when the window was completely painted, probably by 
adding a boolean to the damage event to pass that information along.

However, when you create a Damage structure for a window, it is 
initialized to the borderClip for that window; if the window is mapped 
(and it often is, even for new windows because applications are pretty 
quick to create/map menus) then the whole window ends up damaged, even 
though it hasn't painted anything yet.  I don't see how to fix that --
the new Damage would have no context to know whether the bits in the 
window were painted by the application or not.

However, we can easily solve case 2) -- just copy the screen area to the 
backing pixmap when the window is mapped.  If the compositing manager is 
doing anything weird with the windows, you may get strange looking results 
though -- any shadows will be painted around the "window", even though 
that "window" was filled with bits from the screen underneath it.

I guess the right solution is to implement the trivial fix for 2) and then 
work on ICCCM extensions for applications to communicate with the 
compositing manager about window painting state.  Those extensions are 
required to avoid screen artifacts in other cases, so I guess it's not so 
bad that we need them for menus as well.

-keith


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