I have encountered a small error in window management of KDE5. As I tried to move windows around by grabbing from the middle of the title bar, I noticed that that usual operation didn’t go smoothly. Sometimes my attempt was ignored, sometimes succeeded after holding the left mouse button long enough until the pointer’s icon changed. The quick shuffling of several windows to create a comfortable workspace was impossible.
I found a solution by disabling two features: expansion of window when it is pulled to the upper edge of the desktop, and expansion with double click on the title bar.
Now works normally. Logical error, I guess …