I have also this message on Linux (ubuntu 20.04) , with 3.0.beta1
As pixman is not part of gnome (nor otherwise glib user), even #44995 won't help in debugging this. Likely requires building quite a bunch of freeciv dependency libraries yourself.
Built just pixman, and it was enough to get this tail of backtrace (from S3_0 + several local patches):
(function=function@entry=0x7ffff33e8e50 <__PRETTY_FUNCTION__.11> "pixman_region32_init_rect", message=message@entry=0x7ffff33e8b37 "Invalid rectangle passed")
#1 0x00007ffff33b28a0 in pixman_region32_init_rect
(region=0x555557914668, x=<optimized out>, y=<optimized out>, width=<optimized out>, height=<optimized out>) at ../../../src/pixman-0.42.2/pixman/pixman-region.c:389
#2 0x00007ffff6dca91f in cairo_region_create_rectangle () at /lib/x86_64-linux-gnu/libcairo.so.2
#3 0x00007ffff6f49ffe in () at /lib/x86_64-linux-gnu/libgdk-3.so.0
#4 0x0000555555655ff9 in dirty_rect
(canvas_x=canvas_x@entry=434, canvas_y=canvas_y@entry=867, pixel_width=pixel_width@entry=126, pixel_height=pixel_height@entry=-35) at ../../../src/client/gui-gtk-3.22/mapview.c:437
#5 0x00005555555e629b in update_map_canvas (canvas_x=434, canvas_y=867, width=126, height=-35)
#6 0x00005555555e7570 in movement_animation (time_gone=<optimized out>, anim=0x55555718be10)
#7 update_animation () at ../../src/client/mapview_common.c:420
#8 0x0000555555655f00 in map_canvas_draw (w=<optimized out>, cr=0x555555e3a110, data=<optimized out>)
Split bug about Battle Animations size -> #47705 - That seems to explain only small percentage of these warning messages, keep this ticket for the majority of them.
Patch for all branches, including S2_6.
The following error messages appears on the console during a Freeciv S2_6 game:
I saw 10 copies of this message during a test game of about 50(?) turns, lasting 20(?) minutes.
Freeciv version: S2_6 branch, commit 9cd08cd4cbe3dc201703e3609d42723f69065cca .
Client: freeciv-gtk3.22, "Freeciv version 2.6.4+ gui-gtk-3.22", on macOS 10.13.6 High Sierra
Game: local server, Civ2Civ3 rules, AI level Normal, Hexagonal tiles, no North-South wrapping, otherwise default settings.
From a little bit of web searching, I suspect that _pixman_log_error might be a GTK3 name. I do not find that name in a search of the S2_6 branch source code.
Sorry, this not a well-isolated bug situation. I just want to get a placeholder into the bug list, for future refinement and isolation.
Freeciv code never passes an invalid rectangle to void pixman_region32_init_rect(region_type_t *, int, int, unsigned int, unsigned int). No messages like this are printed to the console during games.