Breadcrumbs toolbar doesn't scroll when in the middle of trying to drag&drop a file
(tested with Thunar 4.16.10 on a live session of Mint 21.0 Xfce as well as Thunar 4.16.11 on a live session of Manjaro 21.3.6)
Simply put, when you are in the middle of trying to drag-and-drop a file into Thunar, if you hover your mouse cursor over the scroll buttons located on the left and right side of the breadcrumbs toolbar, the breadcrumbs toolbar does not scroll.
This is most easily confirmed by just shrinking your Thunar window to the point that only the current directory is listed on the breadcrumbs toolbar.
For reference, at least Caja v1.26.0 and Nemo v5.4.2 do in fact properly scroll the breadcrumbs toolbar if you hover your mouse cursor over the scroll buttons located on the left and right of the breadcrumbs toolbar when in the middle of a drag-and-drop operation.