Which version of the software are you running? It doesn't make too much difference to the solution but would be good for Mentor to know.
If you right click at the top of the frame (even without the title bar) you should see an option to 'Allow docking' , deselect this and drag the window. If you cannot see the context menu on the mouse click then you can reset the windows by deleting the DxDesigner.wsp file from your WDIR directory. This will reset all windows to default so is a bit of a sledgehammer.
The problem was solved by deleting the DxDesigner.wsp file！Thanks very much！
P.S.: My version is EE7.9.3.