I can confirm that libgtk-3-0 3.24.20-0ubuntu1.1 from focal-proposed fixes the rendering issue.
I have tested with:
1) the attached minimal test case:
gcc -o testcase testcase.c `pkg-config --cflags --libs gtk+-3.0` ./testcase
2) a full-blown flutter app:
snap install --classic flutter flutter config --enable-linux-desktop flutter create test_app cd test_app flutter run
I can confirm that libgtk-3-0 3.24.20-0ubuntu1.1 from focal-proposed fixes the rendering issue.
I have tested with:
1) the attached minimal test case:
gcc -o testcase testcase.c `pkg-config --cflags --libs gtk+-3.0`
./testcase
2) a full-blown flutter app:
snap install --classic flutter linux-desktop
flutter config --enable-
flutter create test_app
cd test_app
flutter run