Created attachment 556421 [details] testcase Press the 'Open' button in the testcase, then resize the main window using the resizer above the 'Open' button. The panel moves in the opposite direction of the resize. It should stay put. This is Mac specific.
nsChildView::WidgetToScreenOffset is returning a y value one pixel too high as the main window is resized, causing the popup to adjust its relative position because it thinks the window has moved. But since the main window hasn't moved at all, the popup's relative position get adjusted by one pixel. The returned y position of the window shouldn't change when the window is moved. Since this doesn't happen horizontally, I suspect something to do with the flipped cocoa screen coordinates, and using a change to the bottom edge of the window instead of the top edge. Note that you can either use the <resizer> element in the window or the normal mac window corner to resize, and the same issue occurs.