Expecting distro maintainers to test every GUI feature on every piece of software on Linux is a tad unrealistic don’t you think?
You’d expect (hope) core applications would be tested but what about stuff that’s not in the official repos? Or stuff that is but had a new feature unbeknown to the distro maintainers?
On HN last week there was a GUI bug in a core OSX utility being discussed that slipped through Apples testing. If they managed to overlook something in their own OS, then a smaller team of people, likely working for free in their spare time (remember a lot of the distros that ship non-standard themes as a default are spin offs) is unlikely to test every GUI form and dialog that can be rendered o. Linux.