When you start the 1.9 client, if you decide to go to fullscreen and then exit the full screen it locks the window size and you are no longer able to adjust the window size unless you want to play in full screen.
Included is the before full screen (Bug 1)
and
After full screen (Bug 2)
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[media][media]

Can confirm this is the case. I switch between fullscreen and windowed all the time.

Dupe of MC-68754