In the last few days a new and confusing feature has arisen in my Poser 11. If I click on the background, previously, nothing happened, but now, the screen display enlarges - - but the background does not enlarge, thus spoiling a stage setup that depends on things in the background image. The only remedy is to undo the click that caused the magnification, if I notice in time that a magnification event has happened.. Please, how can I specifically set the magnification to 1, i.e. no enlargement or shrinking?

I've never seen such a thing happen. A before-and-after picture would be very helpful, for these sorts of help queries, so we can work out what you're talking about.

You make it sound like the entire "screen display" of your PC is somehow enlarging. But I'm guess it's just the camera in the Poser Viewport, zooming into the scene. 

One possibility would then be that you have a small dome environment for a 360-degree background, and when you click on this Poser is automatically centring your camera to the centre of the dome?

Is your preview display a floating palette or docked  As HartyBart posted, more information is needed to ascertain what is exactly happening.  You really should include screencaps to illustrate the problem, your textual explanation is clearly not detailed enough to understand.

That .pz3 file do not have a 360deg dome environment, But I have called it again just now and the "magnification on clicking on the environment" did not happen. But I have noticed that from time to time a program can get into a state when it thinks that a key is down when that key is actually up.

Have you got an FBX in scene ? I've had unrepeatable problems with their 'Imports' stepping on other parameters. Typically when multi-rigs ensue, a shade short of 'splat'...

Bit like those weird cross-talk effects with earlier Poser versions that were famously tamed and trained...