3D Viewport Pan and Zoom buttons stopped working?

( LATEST: I closed the LH window by sliding the RH window left, and when set to display the 3D viewport the zoom and Pan buttons worked normally, so it does look like a bug, with some forensics contained in the attached file. END.)

I would be grateful if, before I report a bug, someone could check if they get the same problem in this blender file?

cubes falling.blend (798.0 KB)

Description of Problem:
In the above file, select the left-hand ‘window’ and by dragging on the magnifier or hand icons (‘buttons’) try to zoom or pan. I find they do not now do anything. These both work on the right-hand window.

Pan and zoom were working fine up to this point. I was trying to position the camera by adjusting the camera itself in the LH window using Layout mode and Object view, while checking the the Camera View in the RH window.
Suddenly the mouse would not work on the two Pan and Zoom ‘buttons’.

I am using the April 2 build, on a mid-2011 MacBook which works fine.

The scene is a simple test layout of two cubes and a sphere for exploring physics animation. Animation and a render worked OK.

BTW When I checked the uploaded file, it seemed to open at the render viewer, just select the main Blender window.
Thanks in advance.