XDamage extension over-reports window damage

[Keith Packard]

Around 15 o'clock on Jan 26, Jonathan Lennox wrote:

> Specifically, I've noticed two problems.  First of all, Xdamage reports
> damage if a window *obscured* by the one I'm monitoring is drawn to.  E.g.,
> if I have output going to an XTerm that is partially obscured by the window
> I'm monitoring, the area of my window that covers the XTerm continually gets
> damage reports.

I think this can probably be fixed; the limitation here is that damage 
monitoring occurs only on the target holdin the physical pixels; when two 
windows share the same physical target (e.g. the screen), then damage 
monitoring on one will see damage done to the other; the trick is to clip 
the damage so that it doesn't impact the underlying window.

It can't be fixed in all cases; the damage inside the server is reported 
in rectangular units, so some (odd) drawing will still "leak" through to 
the underlying window.

> In this case, with raw or delta damage report types, it seems like
> damage reports should be sent just for the area of fvwm2's trim, but instead
> I get a rectangle encompassing the entire window. 

Again, damage is internally recorded in rectangular units, so an operation 
which ends up reporting only a single large rectangle can appear to damage 
the internal window contents.

The guarantee Damage provides is that you will get notification whenever 
the contents of the window changes; it doesn't guarantee that you will get 
minimal notification.  Changes to the server which reduce overreporting 
would be gratefully accepted.

-keith


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