If I apply a filter that causes no matches, nothing is done to the repo tree.
The only visible notification appears to be the "0 matches found" message in the status bar, which is quickly cleared out by me clicking in the tree.
In my case, I was trying to to tell if a revision had been merged into a branch or not (e.g. "ancestors(branch('stable')) and f88b2ec8c2ee" in the thg repo).
I think it would be more consistent and useful if the tree was filtered even if there were no matches found.