Dragging video canvas in non-fullscreen-mode

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If I grab the video canvas with my mouse and try to drag the window somewhere else, it messes up the position of the window. After I do that, the video keeps jumping away from my mouse (till I restart the app). If I try to move the window via the tabs (above the video canvas) it works perfectly.

EDIT: It only happens when I try to grab the top of the video canvas. Below a certain level it works fine. I also noticed a small jump of the window right before I start moving the window (below that certain level I described). It looks like the grab coordinates differ (are around 50px higher) from the actual mouse coordinates. When it goes wrong it looks like it is thinking I am still dragging, while I did already released the left mouse button. Though it is only pushing the window in one direction (e.g. down and to the right, not up and the left) result: the window leaves my screen and I am unable to bring it back.

No files or tests are named in the report. First reproduce the problem by dragging the top of the video canvas in non-fullscreen mode, compare it with dragging the tabs or lower canvas, and trace the relevant window-drag behavior; done means the window remains under the mouse after release and can no longer jump off-screen.

Written by the indexing model from the issue text.