Sorry to leave this hanging - this turns out to be a software bug, the Speed Download Lite Safari plugin and the app itself's background handler would periodically shaft the whole computer in dramatic and impressive manner. Ever since I disabled the plugin, and set the app to self-quit after downloading things, no more problems at all.

Meanwhile, Yazsoft say nary a word in response to my various enquiries about this. Pff.

As to my OP - originally it appeared to be only happening to an image window opened in CS4 - but then it suddenly happened to a Safari window too, so I realised it wasn't a CS4 issue and crossed out that bit. Not clear communication on my part - what they had in common was an open window going black, not the editing part (and then sometimes the entire desktop would black out too).

It's very educational just how much havoc one little app's plugins can cause.
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If it's brokenless, don't suffix it...