Thank you for your video.
It seems this only occurs if the Editor is NOT fullscreen. 🙈
When i maximize the Editor window, it does not happen.
When the Editor window is not maximized i can create a brush and drag+drop of materials does not work.
To fix it, i can either
change the view layout
change the size of the 4 views (by dragging the "cross" between views.
resize or maximize the editor window.
All that only fixes the issue when the Brush is already created. If i do that before i create a brush, the issue is there
(Unless its maximized initially, which probably works only because the maximized state is saved, the editor window size is not saved.)