This commit caused this regression: https://git.xfce.org/apps/xfce4-taskmanager/commit/?id=be159c9
Created attachment 9697 diff
Andre Miranda referenced this bugreport in commit 6c78030661f9a490650cfda7a5b48e3c55f6076e Fix closing taskman with Esc (Bug #16656) https://git.xfce.org/apps/xfce4-taskmanager/commit?id=6c78030661f9a490650cfda7a5b48e3c55f6076e
Fixed with the two subsequent commits.