In 4.9+ xfce4-panel the icon-like plugins typically occupy 1 row to allow multi-row panels/deskbars.
This is done by setting "small" property to "TRUE" (default "FALSE") and size requisition to panel size / nrows.
Created attachment 4105
A patch implementing this feature.
Includes panel version checking for compatibility with 4.8.
Created attachment 4121
A patch implementing this feature (rev.2)
- compatible with libxfce4panel < 4.8 (in case if needed)
- corrected button size.
Created attachment 4438
Ok, I could have saved me some work by finding this bug earlier. My solution is only slightly different in that it does not send an additional (unneeded?) size request.
That was one of the earliest plugins I patched (hence the ugly macros).
But AFAIR button/icon size was a bit off if I didn't set the button size request. I didn't bother checking what's going on (an unrelated bug, or just my bad day?), setting the button size request request fixed that.
Fixed in commit f0e810321ef39e0ab57b567469b3785626f7cffc.