New dragged layer should come uo in their original position if Both the document have same pixel dimensions
otherwise center of the document window (if the source and destination images have different pixel dimensions).
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Yes, I'm fully aware of the different behaviour of the Shift-drag procedure occasioned by different pixel dimensions in the two images - but there's still a difference in behaviour between PSE10 and earlier versions which causes my problem.
In the olde versions, SHIFT-drag only worked properly (for different image sizes) if the whole destination image was visible in the window. By pressing View->Fit Image in Window I could achieve this. However, in the 2-up mode in PSE10, View->Fit Image in Window no longer seems to do that (at least, not with my images - maybe it's something to do with the image size, about 5000 pixels wide). Consequently the SHIFT-drag method doesn't centre the layers as I would expect.
So the workaround I've discovered is to use CTRL-minus to reduce the displayed image size until it's fully visible, whereupon SHIFT-drag works as before. I suspect the Window->Images->Float All Images option is the equivalent of the old unMaximise, but for some reason it's greyed out, along with Tile and Cascade (both of which I used to use a lot). I guess this functionality has been removed from PSE10.
To enable the window > images > float all images .Go to Edit >> preferences and select "Allow flaoting documents " option .
It will allow you to make floating documents which is not sticky to the application .
(not sure this is same as old unMaximized option.)
Aha! Bingo! Yes, it's very similar to unMaximise, though the windows float over the whole screen rather than just within the editing area. However, it restores View->Fit Image in Window, and hence the expected behaviour of SHIFT_drag, so many thanks. Works for me!