Expand window to free space on screen in KDE
So, I wanted to avoid the classic "someone else had this problem but no one has answered this" google result. I'm not an Ubuntu/Kubuntu user but I found that the behaviour you desire has been in the bug reports for almost 9 years for KDE. It turns out that with the way KWin is written it would be very difficult to implement (or perhaps no devs want to tackle it).
Anyway. Here's the bug report for any future googlers who come by:
https://bugs.kde.org/show_bug.cgi?id=78984