I'm working on a GUI app in Qt where we use X11 in several places. The main usage is to open multiple connections to the X server for GL widgets, so that we can refresh their time-sensitive contents even when the main Qt event loop is blocked. Another is to query the current X11 virtual desktop as soon as possible after app startup, so we can display subsequent Qt windows there by default even if you switch away to browse the web while our levianthanic app loads. There are a number of less interesting examples, like setting X11-specific attributes on windows, and papering over minutiae that varies between e.g. X11 and OSX. Despite these few things, Qt has served us fantastically well.