Created attachment 102790 [details] screenshot Mac OS X, Multiple document mode set to Subwindows. Installed from .dmg package. Affects 3.0.x version too. When you maximize document subwindow it's title and border are still visible and occupy space.
Hi, Yes... I know -- it's a problem in Qt, our development platform, that we cannot influence. It should be reported to https://bugreports.qt.io