Unintentional, considering I'm on Windows right now.
FOSS GUIs tend to just leave a lot to be desired quite a lot of the time, and in particular will disrespect theming within the OS (or the user's choices) inconsistently - which is quite a major annoyance for a paranoid twat like me who likes things to be consistent. Once you've decided to actually optimise it for 'whatever' platform so you respect its general conventions in GUI you've bloated it up quite a bit.
(There's a reason, for instance, that K-Meleon, an old, long-dead FF fork actually came about: it was because the dev realised that drawing the UI functionality with native Win32 functions was far more optimal than Mozilla's XUL language. Another great example of a FOSS project that uses native UI functionality to make the application feel more 'at home' is the MacOS-exclusive OpenEmu, an implementation of libretro which I remember spinning up on a MacBook Air years ago - RetroArch itself just felt alien on that computer in comparison, I'm gonna be honest.
Another example I could use is how 'out of place' UWP applications feel in Windows 10, they feel 'tacked on top' of the Win32 API, alongside everything else added post-Windows 8 Consumer Preview...)
At least 'cross-platform' these days doesn't tend to mean something like Java though, thank fuck. Back then a different runtime environment could cause chaos (and then there was Microsoft doing their weird shit with the JVM after actually writing a pretty good implementation that beat Sun's own one by quite a bit).
I don't work in software dev, I'm just an end user, but I hope what I'm saying makes sense.