Sometimes Glyphs switches to a different desktop when using a plugin. I think it is switching to the desktop that was open when Glyphs was started. After an open file has been drag and dropped to another desktop, Glyphs will switch back to an empty instance of Glyphs on the desktop Glyphs was started on. For example, open a file. Drag and drop it to another desktop. Activate Offset Curve. The offset curve dialog box will open on the first Glyphs desktop instead of appearing over the desktop where Glyphs is actually being used. I think that this can also be caused by expanding to fullscreen too many times but it’s hard to reproduce that.