I think you should show a flat list of matching items. There really isn't anything else you can do, and this solution has the obvious downside of losing the hierarchical "context", but when you design things, you're always making trade-offs. -tranglos
I wondered if the modern preference for live search, showing a flat list, might eat away at the older tree design... Also about an optional display mode showing the hierarchical context rather like XYplorer's Mini Tree, which shows only the branches of the directory tree that you used recently. In this case, showing an expanded version of the branches that contain matches, with the nodes that individually don't contain matches not so much greyed out as "lowlit" in some way. But I don't know how easy that would be to program

Whether it's instant search or a "press Enter to go" filtering, it's a very powerful feature that helps you narrow down the scope of what you're looking at and concentrate on the results.-tranglos
I still like to see full Boolean searching, because of my work history of searching Derwent World Patent Index on the Dialog host system. I've mentioned this in the thread
NoteFrog Pro (clipboard information manager) - Mini-Review.
In fact, apps like RightNote (all serious note-takers) need instant search (or filtering) in two places: not just for the text of the notes, but for the tree as well. I have a couple of KeyNote files with thousands of nodes in the tree (archives of code snippets, for example). It's next to impossible to locate one node among so many; even harder to find multiple nodes that would match some imagined filter.-tranglos
That much data reinforces my argument in favour of Boolean searching

I only wish I had realized all that 12 years ago 
-tranglos
I never much used KeyNote (which I discovered late), because I didn't think it had searching powerful enough

In RightNote and other database-backed applications there is the problem of performance. You just can't beat direct memory access for speed. With a large database, no matter how indexed, a true instant search may not work very well, as the search will introduce a perceptible delay. So it might be perfectly OK to wait until the user finishes typing the search terms and hits Enter - that way there is only one small delay instead of many (in-between pressing each key).-tranglos
NoteFrog works as an array in memory.
I sort-of agree about speed, but with Windows as slow as it is, isn't everybody conditioned enough to waiting?

And if your data set is big enough, won't it be database time anyway?