I have a configuration with a laptop + an external monitor. In this dual monitor config I have configured a top panel and a bottom panel on both monitors (the proposed configuration of a top panel spanning all monitors was too bad: notifications and workspace switcher were on the far right of the external monitor, thus far away from my mouse pointer when I was working on the laptop monitor). I often have to give presentations or goto to a meeting with my laptop. Therefore I often switch to single monitor mode. In such a case, both my top panels end up on top of one another on the single monitor. This makes them totally unuseable.
I am having the same problem. I have 3 monitors and 3 panels. For each panel, the correct monitor is explicitely selected as output. When I disconnect the two external monitors, their panels become hidden. However, some time after this (possibly after multiple suspend cycles), these panels reappear, all stacked on top on my remaining monitor. The "Output" dropdown box has also become grayed out. xfce4-panel -r restarts the panel and restores proper behavior (including hiding the two panels again and making the "Output" box accessible again.
Ok, now I can confirm that suspending triggers this behavior.
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