I posted this previously last August, no reply from JRiver people, so I'm not optimistic.
I use JRiver everywhere: Windows, Mac, etc. and really love the program. Surely someone else has had this issue, which makes using JRiver on Mac pretty exasperating.
When you left-click on the mini player window to drag it around on the screen, the window grows uncontrollably in both x-y dimensions. Just now, dragging it around, it seems like, if you drag the thing long enough, it will totally fill the screen. Just dragging it a few millimeters in a typical re-positioning action on the screen, it grows quite a bit, making even minor re-positionings of the window quite exasperating. After you drag it a tiny bit, you have to try to pre-judge things and then laboriously make it smaller from the side and bottom since selecting the window in the lower right corner for shrinking the window in both dimensions is virtually impossible to achieve. Please look at this - I can't quite believe nobody else has the issue. I had it before Big Sur on two different MacBook Pro's and on a Mac Mini. It's just as bad or worse in Big Sur. This is really, really an irritation although not a functional, listening problem. (This issue does not exist in Windows.) Thanks.