Scrolling through window buttons is broken

Consider the following window constellation with grouping by application enabled:

| (2) Thunar | Mousepad |

If the Mousepad window is active, I can scroll up to reach one of the Thunar windows, but not the second one.

If I then scroll down, I will reach the second Thunar window, but then I cannot escape the group anymore. Any further scrolling down just switches back and forth between the two windows of the Thunar group.

Package version:

• xfce4-panel 4.18.0

#691 might be related to this.