Window wouldn't shrink to one monitor

I have two monitors; the left one is the main and the right is a desktop extension. I have the montage window on the left (not maximised) and the control window (with master section and meters) on the right.

I loaded the montage I was working on yesterday, then replaced the file with a new version that had been noise-reduced in another program (iZotope RXii). Then I rendered the CD tracks, specifying to open a new montage. When the render completed, the montage window expanded itself to about 1.3 monitors width, and although it could be dragged to increase the width and decrease it again, it would not decrease below this expanded size. Then I selected the CD sub-window instead of the navigator sub-window that was visible in the usual place above the montage (full-width, like in WL6), and then the window could be resized down to fit on one monitor again. Selecting the navigator sub-window again did not cause the problem to recur.