Double-click the Hand tool in the toolbox. Then Choose View > Fit On Screen.
Select a zoom tool or the Hand tool, and click the Fit On Screen button in the options bar.These options scale both the zoom level and the window size to fit the available screen space.
So you want to enable Scrubby Zoom, but it's grayed out in your Zoom Tool's option bar?
Have you updated your graphics drivers lately?
IIRC, scrubby zoom is dependent upon your graphics processor and if PS sees an issue with your drivers, it's one of the features that is auto-disabled by the program.
It was after an update, don't know witch one. I've just fixed It by clicking out 'user opencl' (preferences - performances - advanced GPU settings) and restarting. Sorry, My photoshop is in Dutch. Anyway problem solved for me.
Glad your problem solved