Hi, after unplugging an external monitor it vanishes correctly from the monitor list on the left. However the values on the right side are not refreshed. As a result they still represent the unplugged monitor.
Created attachment 4277 Select a new display when active one is unplugged to correctly update the settings When the active display in the display-settings dialog is unplugged no new display is selected and the display values don't get updated. This patch selects the last display in the list when the active display is removed.
I just tested this patch and applied it in the bluesabre/display-settings branch, which is targeted for Xfce 4.12. Please test it from there, it works for me.
Applied in master (947d77198e8dd63b467023f5a13cbabf322fa943 and b14b00b0d6997b16a89ca0a4ac64860bc71862b3).