I can’t reproduce this reliably, which I realise is no help at all for debugging…
Sometimes when I select an image in the PhotoLibrary the correction preview displayed in the top pane does not scale correctly / fit to the size of that pane - even though ‘fit on screen’ is active. The preview is shown too large, i.e. only part of the image is visible. Changing the size of the correction preview pane sometimes fixes things and the image scales correctly, sometimes I have to resort to changing the zoom level and then resetting ‘fit to screen’ before the image sizes as it should.
I wonder if it’s something to do with the size of the file being previewed as I’ve never seen this odd behaviour with the 10MB files from my Canon 400D and I’ve used PL since v2 (and Optics Pro 8 & 11 before that). This behaviour has only appeared since I got a Canon 90D (30MB files) a couple of weeks ago.
No, even after all the thumbnails in the PhotoLibrary have finished loading, the one showing in the top frame does not fit to screen.
When I started typing this reply, I thought I’d found a way to reproduce the problem. I thought the effect appeared when first image in folder, i.e. the one selected by default when a folder is selected from the tree view, was portrait style but that’s not the case.
The number of images in a folder doesn’t seem to matter either. The only pattern, as I noted in my original post, is that .CR2 files from my Canon 400D are fine (and always have been), this effect only appears when browsing folders holding .CR3 files from my Canon 90D.
I’ll keep experimenting to see if I can come up with more specific details that might help you reproduce what I’m seeing.