The related less annoying problem remains. Surely this is a defect of Synaptics TouchPad V7.5? The action of moving the cursor along the screen is not the same as pinch-zooming.
I no longer have to repeatedly use Ctrl-0 to return to normal size.
I have waited a few days to be sure and can now say with certainty that unchecking the pinch zoom in Control Panel /Hardware and Sound /Synaptics TouchPad V7.5 /Settings solves the problem of the changing of the screen font size.
