Okay I did some more digging and I believe I've fixed it. What a weird bug. In my code I'm querying for the bounding boxes for the title bar and min/max/help/close buttons (via WM_GETTITLEBARINFOEX) and it was reporting that the Close button had a negative width equal to the whole width of the monitor. And then I was marshaling the data wrong, which resulted in a Close button with a positive width equal to the whole width of the monitor. Which means it was "covering" the draggable title bar area
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