Marking this as Solved was premature.

In my current project I usually open two workbooks: the first is one of several hundred 'sources' from which data is being copied by my macro to the other, WalkIndex. The bizarre thing is that opening any source results in it taking exactly the same size as the last size of WalkIndex. And that's true whether WalkIndex is open or closed!

Earlier I always had WalkIndex maximised, so I only discovered this weirdness accidentally when I reduced its size. Position gets copied too, BTW, apart from the usual small offset (23 px here). And now, having just closed both workbooks, I'm once again unable to open any Excel file on my PC.

Any insights would be appreciated please.