When I try to drag window between pager workspaces, panel hides. In order to complete draging, panel must be revealed again by mouse in order to drop window where it should go. Careless droping out of panel triggers another bug, where pager draging stops to work at all, requiring process restart.
Which version of the panel are you using? Are you using auto-hiding or intelligent hiding?
Clarified
This is side panel with permanent auto-hiding. Describing bug in other words, in begining of dragging panel behaves as if freed - it will hide, if must.
Same thing happens when dragging window buttons.
https://git.xfce.org/xfce/xfce4-panel/tree/panel/panel-window.c#n1199 This code part is only executed on button release.
Created attachment 8768 Patch to fix the pager plugin I had written a patch before Theo's comment. I may have to look at that part of the panel code again, as it may be more generic and beneficial to all plugins, whereas I only fixed the pager. In any case, feel free to take a look at the patch - it fixes the problem too.
FWIW the same issue exists in the tasklist plugin (when setting the order to DND).
Simon Steinbeiss referenced this bugreport in commit 95319248145b5506f5a639c7ccdd7732772d74bd pager: Keep panel visible on drag and drop (Bug #15703) https://git.xfce.org/xfce/xfce4-panel/commit?id=95319248145b5506f5a639c7ccdd7732772d74bd
Simon Steinbeiss referenced this bugreport in commit 75a10e232c2ebbd5bd3a04d580f522fbbf3495cd tasklist: Keep panel visible on drag and drop (Bug #15703) https://git.xfce.org/xfce/xfce4-panel/commit?id=75a10e232c2ebbd5bd3a04d580f522fbbf3495cd
I decided to at least fix it for the pager and tasklist to have it working for 4.14.