(aseprite version 1.2.9) When you go to drag a Dialogs using the mouse under Ubuntu (most recent version, I stay up to date daily) causes the Dialogs (any Dialogs in Aseprite) to move erratically and never as expected while locking the mouse until the Dialog stops moving.
When you click to drag the Dialog, it moves initially in the right direction for a split second but then begins to move in the opposite direction. If you keep the mouse button down and move the mouse the window may jump around but continues to move in the wrong direction. if you let go of the mouse button the Dialog continues to slowly move around, depending on how much you tried to move the window, and you are stuck waiting for the Dialogs to stop before you can do anything because the mouse is locked until then. This happens with all Dialogs that aseprite opens up like the preview Dialog, etc. I have had this issue since I bought aseprite back in August.