Running Ardour 6.3.0 on (X)Ubuntu Studio 18.04, windows are bigger than my screen and so some controls are unreachable. My standard resolution is 1366x768. I have seen the problem outside of Ardour too, which may indicate that the problem resides with my installation, but most programs work as expected.
I prefer to work with maximised workspace. When starting Ardour the Editor window starts correctly at the top, but continues outside the bottom of my screen. To fix it I have to deselect and select again View > Maximise Editor Space.
However, that does not prevent dialog windows from overflowing. For example, opening the export dialog and then clicking to ‘Edit Export Format Profile’, the ‘Save’ and ‘Cancel’ buttons of that window are off screen.
I am aware of this post Ardour 4.7 window size weirdness but opening a new topic since quite a few things have changed since Ardour 4.7