Since updating to Yosemite, Finder has been acting strangely. I can usually get it to show a normal-looking and -behaving window for a few operations, and then the window will either:
Be blank, showing no files or folders where I know it is pointing at a non-empty folder.
Show prior state, rather than the state I know to be true now. For instance, I could delete a folder, but it still shows as present, even after forcing Finder to refresh the view somehow.
Double the labels under the finder item:
The only third-party extensions showing in the "Extensions" system pref pane are Dropbox and PCalc.
I've read that old versions of Rogue Amoeba's "Instant On" feature can cause Yosemite problems, but I'm on the current version, 8.0.2.
I can fix it temporarily by dropping to the Terminal and saying