Allow preserving the opacity of the last focused window of each monitor

Submitted by Steve Dodier-Lazaro

Assigned to Olivier Fourdan @olivier

Link to original bug (#5855)

Description

Hello,

When enabling the compositor and setting the inactive windows' opacity < 100%, I can notice that the last focused window of each of my monitors gets drawn with transparency, which prevents me from reading them (since, as the last focused ones, they are on top of my screen and I want to be able to read them).

I would instead expect to be offered the possibility to keep the last focused window of each workspace as opaque as the active one.

This could be integrated with a GtkCheckbox in the opacity settings dialog, just under the slider for inactive windows opacity, titled "Exclude the last focused window of each of my monitors".

I would be ok to try to write a patch for this, but I'd love you to point me to where the list of windows to be drawn with transparency is set, and I'd also need to know if xfwm keeps a list of windows somewhere, or if I have to grab it through Gdk (which could be slow if this is called on each focus change). I suppose it would be more efficient to keep a list of last focused windows per monitor somewhere, but I don't know if there is a preferred place for that in xfwm.