Created attachment 6150 Patch for the issue I've already sent this patch to the xfce4-dev mailing list, but things seem to be going slow. So I'll repeat it here, just in case. --- This patch fixes counter-intuitive behavior for window buttons in deskbar mode.
The problem that is solved by the patch: When you set panel mode to 'Deskbar', and window buttons sorting order to 'None, allow drag-and-drop', the behavior of drag&drop is counter-intuitive. You have to drop button on left or right side of other buttons, like in horizontal mode, instead of between buttons, like in vertical mode. Patch above provides simple fix for that problem.
Created attachment 6577 Fix for oversight
Created attachment 9330 Fix for the oversight Bug is still present in 4.14. Updating the diff.
Created attachment 9332 Demonstration of undesired behaviour
Re-created it here: https://bugzilla.xfce.org/show_bug.cgi?id=16298