This is discussed in this thread: https://discourse.panda3d.org/viewtopic.php?t=1313
If you use messenger.toggleVerbose(), you’ll see that the event is still getting sent as “shift-w”, even though you have issued the base.mouseWatcherNode command.
I believe you have to set the modifier buttons to empty on the ButtonThrower object, as discussed in the referenced thread. That seems to work fine for me.
David