according to the manual, zooming should be possible using ctrl+scrollwheel.
With V3.1.2 64-Bit which I just installed, this seems at first look not to work.
At least is does not work if the mouse cursor is over the document window.
But placing the mouse cursor over the bookmarks window allows to zoom the document window.
Doesn’t seem right to me 
Lorenz
Enhanced Functions such as scroll wheel can vary per user based on hardware
using 3.1.2 on Win 7 x64 , using either a Logitech or Microsoft branded mouse (nearest I get to your setup ?)
I can use ctrl and wheel to zoom view area as described in either bookmark or main viewing area or even outside the window i.e over the adjoining desktop
ODDLY for me using (one handed) right button and scroll wheel only zooms whilst in the main viewing area or outside the window BUT NOT when in the bookmarks area
sorry for the noise. I’ve tracked down the problem and it’s no fault of Sumatra PDF on its own.
I’m using KatMouse (Win7-64 here too) to move the scrollwheel focus to the window under the mouse cursor and it seems KatMouse hooks into the scrollbar control directly (if one is present).
The result
So I reconfigured KatMouse to ignore Sumatra PDF and now zoom works as expected.
Lorenz