compositingActive not efficient on XRandR events

[Fredrik Höglund]

On Monday 18 July 2011, Aaron J. Seigo wrote:
> On Saturday, July 16, 2011 15:36:19 Martin Gräßlin wrote:
> > The bug is much simpler - Plasma just simply fails to recognize that a
> > compositing manager is active. I can see this each time I restart kwin
> > (which is considerable often). In order to get translucent panels back, I
> > have to kquitapp plasma-desktop, plasma-desktop.
> 
> Plasma::Theme uses a KSelectionWatcher which watches _NET_WM_CM_S# where # is 
> the number of the default screen.
> 
> i can imagine a few things going wrong with this:
> 
> * the default screen # changes or even goes away completely; that could render 
> the selection manager useless. why is the CM atom per screen again? *sigh*

Screen here is about traditional multi-head where you can have different
window managers / compositing managers managing each screen.

> * a race condition as Alex outlined. if kwin is indeed responding to each 
> xrandr even with a change in the CM, that seems like a perfect candidate for 
> event compression if at all possible: don't tell the world outside that things 
> have changed until the events have stopped coming in. the timeout for this 
> shouldn't need to be long at all, so the user shouldn't see a big change at 
> all
> 
> * KSelectionWatcher itself and/or kwin's setting of the atom could be broken. 
> in times past we've had isses where the KSelectionWatcher object simply did 
> not emit any signals at all when kwin changed compositing.

I thought this bug was fixed, because I haven't seen it in a long time.
But I realized that I still have a patch applied that I created a couple
of years ago.

It adds a compositingChanged() signal to KWindowSystem and makes
Plasma::Theme use it. It also uses Xfixes instead of KSelectionWatcher
to monitor the selection.

I never committed the patch because Lubos didn't like the idea. IIRC the
reason was that the signal would be emitted before the compositing
plugins are initialized in kwin.

I have attached the diff in case it fixes the problem for those who are
still seeing it.

Regards,
Fredrik

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