In xfce4-settings 4.11.1 an option was added to automatically show the minimal-display dialog, when new display gets connected. The checkbutton to set this option is only visible in the standalone display-settings dialog (xfce4-display-settings) and not in the display dialog of the settings-manger.
The problem is that the Plug content (the widgets included in settings manager) doesn't contain the dialog action bar (with the buttons), and the checkbox is added after that. So for this to work, "alignment1" containing "minimal-autoshow" has to be moved inside "plug-child" container widget in xfce4-settings/dialogs/display-settings/display-dialog.glade file.
This bug has been addressed in 4.11.3.