Freeciv

Does the unit_pic_table exist after gtk_window_destroy()? One would assume that it's part of the gtk widgets tree being destroyed. free_unit_table() relies on unit_pic_table.

The code in gui_main.c routine setup_widgets() that creates unit_pic_table is kind of complex and deeply nested so I may not understand it right, but it includes calls to detached_widget_new() and detached_widget_fill(), which implies that unit_pic_table (and some more widgets) are not owned by the window. I did some testing of my patched version, and in free_unit_table() when gui_up is FALSE (quitting the client and after the call to gtk_window_destroy()) the unit_pic_table is still valid. Which implies that it (and maybe some more widgets) are never freed, which doesn't matter since we're quitting.

I tested with valgrind, and it seems to disagree with you:

The memory might still be untouched, but is in fact freed (and valgrind does catch that)

Also, with the patch there are several occurrences of "1: Gtk: gtk_grid_remove: assertion 'GTK_IS_GRID (grid)' failed" -messages on the console, further indicating that unit_pic_table grid is not ok.

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Valgrind, and other similar tools, are a reason to get freeing memory right even in the program exit, so that the errors don't pollute the reports. It's much easier to spot meaningful entries when there are no scores of false alarm entries to comb through.

Likely you can fix this simply by increasing unit_pic_table reference counter (reffing) somewhere before gtk_window_destroy(), and unreffing after free_unit_table()

Interesting - I do not get any "1: Gtk: gtk_grid_remove: assertion 'GTK_IS_GRID (grid)' failed" messages. Without my patch, do you get the same
"1: Gtk: gtk_picture_set_paintable: assertion 'GTK_IS_PICTURE (self)' failed" messages from set_unit_icon() that started this? If not, then just close this as a Mac-only bug.

I suspect that the gtk4 package I have installed by MacPorts works differently on my Mac than on regular unix, I have seen a bunch of other weird things. Like - "1: Gdk: _gdk_macos_surface_update_fullscreen_state: assertion 'GDK_IS_MACOS_SURFACE (self)' failed" is obviously Mac-only. Or "(<unknown>:10027): Gtk-WARNING *: 21:23:00.230: Trying to snapshot GtkGizmo 0x7f87d96822e0 without a current allocation" - huh? And when I click on the "Ruleset" or "AI Skill Level" buttons I get tons of "2: (<unknown>:10061): Gtk-WARNING *: 21:51:50.275: Broken accounting of active state for widget 0x7fdbb92554e0(GtkApplicationWindow)". Do you ever see these?