On an xfce desktop, when invoked from a bottom panel with a right or left click on a button icon (in systray, app launchers, etc.), many of the menus manifest overlapping the panel rather than just adjacent above the panel as in a fixed always visible panel configuration. In many cases this results in a 'quit' option at the bottom of the option menu manifesting directly under the mouse pointer. If an extra click (or a click-hold-release) passes through one can easily end up inadvertently closing a running application that was docked in the systray when intending to access other options.

Screenshot: http://en.zimagez.com/zimage/xfceiconmenuscreenshots.php

[Linked image shows screenshots of both for comparison with 'autohide' set (top half) and with persistently visible panel (bottom half); while 'onboard' was used as an example, the the same behavior occurs with many, but not all, other icons in panel as well.]

A question arises as to how much of the behavior arises from how individual apps are setup versus how xfce panel is handling them and how the xfce4 panel might be tweaked in future updates to compensate.

Something about the panel state is apparently getting passed by xfce-panel to the various apps and their various toolkits or else there wouldn't be a change in displayed menu placement in the first place, right? The apps which do the erroneous placement appear to be drawing from some variable noting the location of the collapsed state top edge ... if such a variable might be identified and corrected/faked as the location of the top edge when displayed (without app icons detaching from the panel and remaining behind when the panel actually is collapsed) ... It seems at least plausible that some form of tweak to compensate for the vagaries of the app toolkits and such might be handled from the xfce-panel end.

related threads elsewhere:
https://forum.xfce.org/viewtopic.php?id=10591
https://redd.it/4a90k5