When running with MOZ_LOG=WidgetDrag:5 in jammy, this is what I'm seeing when I drag a tab: [Parent 82683: Main Thread]: D/WidgetDrag nsDragService::InvokeDragSession [Parent 82683: Main Thread]: D/WidgetDrag adding target application/x-moz-tabbrowser-tab [Parent 82683: Main Thread]: D/WidgetDrag adding target text/x-moz-text-internal [Parent 82683: Main Thread]: D/WidgetDrag invisibleSourceDragBegin [Parent 82683: Main Thread]: D/WidgetDrag nsDragService::SetDragIcon() [Parent 82683: Main Thread]: D/WidgetDrag set drag popup [7fe9ee004000] [Parent 82683: Main Thread]: D/WidgetDrag nsDragService::StartDragSession [Parent 82683: Main Thread]: D/WidgetDrag nsDragService::EndDragSession 0 The same action with the same version of the snap in impish is successful, and has a lot more logs: [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::InvokeDragSession [Parent 2923: Main Thread]: D/WidgetDrag adding target application/x-moz-tabbrowser-tab [Parent 2923: Main Thread]: D/WidgetDrag adding target text/x-moz-text-internal [Parent 2923: Main Thread]: D/WidgetDrag invisibleSourceDragBegin [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::SetDragIcon() [Parent 2923: Main Thread]: D/WidgetDrag set drag popup [7f944ac8dc00] [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::StartDragSession [Parent 2923: Main Thread]: D/WidgetDrag WindowDragMotionHandler nsWindow 7f9467fd3400 coords [74, 11] [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::Schedule() task eDragTaskMotion window 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::RunScheduledTask() task eDragTaskMotion mTargetWindow 0 mPendingWindow 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::StartDragSession [Parent 2923: Main Thread]: D/WidgetDrag start drag session mTargetWindow 7f9467fd3400 mTargetWidget 7f946abd2760 [Parent 2923: Main Thread]: D/WidgetDrag process motion event [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::UpdateDragAction() [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::SetCanDrop 1 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::ReplyToDragMotion can drop 1 [Parent 2923: Main Thread]: D/WidgetDrag gdk_drag_status() action 4 [Parent 2923: Main Thread]: D/WidgetDrag clear mTargetWindow mTargetWidget and other data [Parent 2923: Main Thread]: D/WidgetDrag remove task source [Parent 2923: Main Thread]: D/WidgetDrag WindowDragMotionHandler nsWindow 7f9467fd3400 coords [55, 11] [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::Schedule() task eDragTaskMotion window 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::RunScheduledTask() task eDragTaskMotion mTargetWindow 7f9467fd3400 mPendingWindow 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::StartDragSession [Parent 2923: Main Thread]: D/WidgetDrag start drag session mTargetWindow 7f9467fd3400 mTargetWidget 7f946abd2760 [Parent 2923: Main Thread]: D/WidgetDrag process motion event [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::UpdateDragAction() [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::SetCanDrop 1 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::ReplyToDragMotion can drop 1 [Parent 2923: Main Thread]: D/WidgetDrag gdk_drag_status() action 4 [Parent 2923: Main Thread]: D/WidgetDrag clear mTargetWindow mTargetWidget and other data [Parent 2923: Main Thread]: D/WidgetDrag remove task source [Parent 2923: Main Thread]: D/WidgetDrag WindowDragMotionHandler nsWindow 7f9467fd3400 coords [51, 11] [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::Schedule() task eDragTaskMotion window 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::RunScheduledTask() task eDragTaskMotion mTargetWindow 7f9467fd3400 mPendingWindow 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::StartDragSession [Parent 2923: Main Thread]: D/WidgetDrag start drag session mTargetWindow 7f9467fd3400 mTargetWidget 7f946abd2760 [Parent 2923: Main Thread]: D/WidgetDrag process motion event [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::UpdateDragAction() [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::SetCanDrop 1 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::ReplyToDragMotion can drop 1 [Parent 2923: Main Thread]: D/WidgetDrag gdk_drag_status() action 4 [Parent 2923: Main Thread]: D/WidgetDrag clear mTargetWindow mTargetWidget and other data [Parent 2923: Main Thread]: D/WidgetDrag remove task source [Parent 2923: Main Thread]: D/WidgetDrag WindowDragMotionHandler nsWindow 7f9467fd3400 coords [51, 11] [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::Schedule() task eDragTaskMotion window 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::RunScheduledTask() task eDragTaskMotion mTargetWindow 7f9467fd3400 mPendingWindow 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::StartDragSession [Parent 2923: Main Thread]: D/WidgetDrag start drag session mTargetWindow 7f9467fd3400 mTargetWidget 7f946abd2760 [Parent 2923: Main Thread]: D/WidgetDrag process motion event [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::UpdateDragAction() [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::SetCanDrop 1 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::ReplyToDragMotion can drop 1 [Parent 2923: Main Thread]: D/WidgetDrag gdk_drag_status() action 4 [Parent 2923: Main Thread]: D/WidgetDrag clear mTargetWindow mTargetWidget and other data [Parent 2923: Main Thread]: D/WidgetDrag remove task source [Parent 2923: Main Thread]: D/WidgetDrag WindowDragMotionHandler nsWindow 7f9467fd3400 coords [47, 10] [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::Schedule() task eDragTaskMotion window 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::RunScheduledTask() task eDragTaskMotion mTargetWindow 7f9467fd3400 mPendingWindow 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::StartDragSession [Parent 2923: Main Thread]: D/WidgetDrag start drag session mTargetWindow 7f9467fd3400 mTargetWidget 7f946abd2760 [Parent 2923: Main Thread]: D/WidgetDrag process motion event [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::UpdateDragAction() [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::SetCanDrop 1 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::ReplyToDragMotion can drop 1 [Parent 2923: Main Thread]: D/WidgetDrag gdk_drag_status() action 4 [Parent 2923: Main Thread]: D/WidgetDrag clear mTargetWindow mTargetWidget and other data [Parent 2923: Main Thread]: D/WidgetDrag remove task source [Parent 2923: Main Thread]: D/WidgetDrag WindowDragMotionHandler nsWindow 7f9467fd3400 coords [44, 10] [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::Schedule() task eDragTaskMotion window 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::RunScheduledTask() task eDragTaskMotion mTargetWindow 7f9467fd3400 mPendingWindow 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::StartDragSession [Parent 2923: Main Thread]: D/WidgetDrag start drag session mTargetWindow 7f9467fd3400 mTargetWidget 7f946abd2760 [Parent 2923: Main Thread]: D/WidgetDrag process motion event [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::UpdateDragAction() [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::SetCanDrop 1 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::ReplyToDragMotion can drop 1 [Parent 2923: Main Thread]: D/WidgetDrag gdk_drag_status() action 4 [Parent 2923: Main Thread]: D/WidgetDrag clear mTargetWindow mTargetWidget and other data [Parent 2923: Main Thread]: D/WidgetDrag remove task source [Parent 2923: Main Thread]: D/WidgetDrag WindowDragMotionHandler nsWindow 7f9467fd3400 coords [41, 10] [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::Schedule() task eDragTaskMotion window 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::RunScheduledTask() task eDragTaskMotion mTargetWindow 7f9467fd3400 mPendingWindow 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::StartDragSession [Parent 2923: Main Thread]: D/WidgetDrag start drag session mTargetWindow 7f9467fd3400 mTargetWidget 7f946abd2760 [Parent 2923: Main Thread]: D/WidgetDrag process motion event [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::UpdateDragAction() [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::SetCanDrop 1 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::ReplyToDragMotion can drop 1 [Parent 2923: Main Thread]: D/WidgetDrag gdk_drag_status() action 4 [Parent 2923: Main Thread]: D/WidgetDrag clear mTargetWindow mTargetWidget and other data [Parent 2923: Main Thread]: D/WidgetDrag remove task source [Parent 2923: Main Thread]: D/WidgetDrag WindowDragMotionHandler nsWindow 7f9467fd3400 coords [40, 10] [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::Schedule() task eDragTaskMotion window 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::RunScheduledTask() task eDragTaskMotion mTargetWindow 7f9467fd3400 mPendingWindow 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::StartDragSession [Parent 2923: Main Thread]: D/WidgetDrag start drag session mTargetWindow 7f9467fd3400 mTargetWidget 7f946abd2760 [Parent 2923: Main Thread]: D/WidgetDrag process motion event [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::UpdateDragAction() [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::SetCanDrop 1 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::ReplyToDragMotion can drop 1 [Parent 2923: Main Thread]: D/WidgetDrag gdk_drag_status() action 4 [Parent 2923: Main Thread]: D/WidgetDrag clear mTargetWindow mTargetWidget and other data [Parent 2923: Main Thread]: D/WidgetDrag remove task source [Parent 2923: Main Thread]: D/WidgetDrag WindowDragMotionHandler nsWindow 7f9467fd3400 coords [38, 9] [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::Schedule() task eDragTaskMotion window 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::RunScheduledTask() task eDragTaskMotion mTargetWindow 7f9467fd3400 mPendingWindow 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::StartDragSession [Parent 2923: Main Thread]: D/WidgetDrag start drag session mTargetWindow 7f9467fd3400 mTargetWidget 7f946abd2760 [Parent 2923: Main Thread]: D/WidgetDrag process motion event [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::UpdateDragAction() [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::SetCanDrop 1 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::ReplyToDragMotion can drop 1 [Parent 2923: Main Thread]: D/WidgetDrag gdk_drag_status() action 4 [Parent 2923: Main Thread]: D/WidgetDrag clear mTargetWindow mTargetWidget and other data [Parent 2923: Main Thread]: D/WidgetDrag remove task source [Parent 2923: Main Thread]: D/WidgetDrag WindowDragMotionHandler nsWindow 7f9467fd3400 coords [37, 9] [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::Schedule() task eDragTaskMotion window 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::RunScheduledTask() task eDragTaskMotion mTargetWindow 7f9467fd3400 mPendingWindow 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::StartDragSession [Parent 2923: Main Thread]: D/WidgetDrag start drag session mTargetWindow 7f9467fd3400 mTargetWidget 7f946abd2760 [Parent 2923: Main Thread]: D/WidgetDrag process motion event [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::UpdateDragAction() [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::SetCanDrop 1 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::ReplyToDragMotion can drop 1 [Parent 2923: Main Thread]: D/WidgetDrag gdk_drag_status() action 4 [Parent 2923: Main Thread]: D/WidgetDrag clear mTargetWindow mTargetWidget and other data [Parent 2923: Main Thread]: D/WidgetDrag remove task source [Parent 2923: Main Thread]: D/WidgetDrag WindowDragLeaveHandler() [Parent 2923: Main Thread]: D/WidgetDrag WindowDragLeaveHandler nsWindow 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::Schedule() task eDragTaskLeave window 0 [Parent 2923: Main Thread]: D/WidgetDrag WindowDragDropHandler nsWindow 7f9467fd3400 coords [37,9] [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::Schedule() task eDragTaskDrop window 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::RunScheduledTask() task eDragTaskDrop mTargetWindow 7f9467fd3400 mPendingWindow 7f9467fd3400 [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::StartDragSession [Parent 2923: Main Thread]: D/WidgetDrag start drag session mTargetWindow 7f9467fd3400 mTargetWidget 7f946abd2760 [Parent 2923: Main Thread]: D/WidgetDrag process drop task [Parent 2923: Main Thread]: D/WidgetDrag drag finished [Parent 2923: Main Thread]: D/WidgetDrag nsDragService::EndDragSession 1 [Parent 2923: Main Thread]: D/WidgetDrag clear mTargetWindow mTargetWidget and other data [Parent 2923: Main Thread]: D/WidgetDrag remove task source [Parent 2923: Main Thread]: D/WidgetDrag WindowDragLeaveHandler() [Parent 2923: Main Thread]: D/WidgetDrag Failed - GetMostRecentDestWindow()! [Parent 2923: Main Thread]: D/WidgetDrag invisibleSourceDragEnd [Parent 2923: Main Thread]: D/WidgetDrag SourceEndDragSession result 0