Follow-up Comment #5, patch #2715 (project freeciv):

Found out interesting facts about GtkPixcomm implementation. Actual drawing
when widget is exposed uses those data pointers that have been earlier passed
to gtk_pixcomm_fill() and gtk_pixcomm_copyto(). This means that GtkPixcomm
users must ensure that color structures or sprites are not freed as long as
GtkPixcomm is in use. This explains at least one bug in my earlier cairo
patches where local GdkColor structure was passed to gtk_pixcomm_fill() but
GtkPixcomm was drawn only after exiting that function.

Corrupted backgrounds in sprites (including animated cursors) seem to be
problem in drawing to gtk widgets. Everything in handling sprites and canvases
seem to work. Only when final draw from some partly transparent cairo surface
to gtk widget surface is made, extra stuff is left to those transparent
places. It looks like widget surfaces had some stuff on them initially and
that's visible in places we don't overdraw.
I actually did test version that had extra level of drawing. Our partly
transparent canvases were drawn on top of another filled canvas so that the
result had no transparent parts at all. That canvas is then drawn to gtk
widget. No corruption was visible with that version, and for a while I
actually considered using it as fallback solution for first canvas using
commit - but then I noticed that even cursors would need that and those really
need transparency.

Improvements in attached version since previous:
- Helpdialog always keeps color-structure for unit GtkPixcomm
- Moved surface initialization outside of the operations iteration when
GtkPixcomm exposed
- Common parts of sprite_get_pixbuf() and canvas_put_pixbuf() moved to
separate function surface_get_pixbuf()
- Completely removed some code that was already commented out in earlier
versions


Buglist:
- Unit sprites not shown at help dialog
- Wrong terrain pieces drawn to main map (mask handling problem?)
- gtk widgets and cursors displays mess where we don't overdraw with solid
color


(file #13849)
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