I have not seen this documented or mentioned, so I figured I'd share this little discovery (even though I'm sure many have come across it already).
And we're now in a hybrid "Power+Thumbnails Mode" of sorts, which is rather cool. 
Too bad "Details+Thumbnails" (or the above 'version') do not really work in Flat View / Grouped mode (all the items are there, but there's no grouping).
Details+Thumbnails is really just Details mode with the Thumbnail column added. You can add the same column to Power mode (and you can add it to Details mode like any other column; it's not really a separate mode, there's just a button for it next to the other modes for convenience).
Flat View (Grouped) works correctly with the thumbnails column:
Maybe it looks strange to you because the thumbnails are not indented, only the filenames? If so, turn off Preferences / File Display Modes / Details / Hide icons when thumbnail column is visible and you might find that better, with the icons being displayed again:
(The Thumbnail column does not have to be next to the Name column, either. You can put it anywhere you like.)
Thanks, Leo, it all makes [even more] sense now!
