Sorry to be recently overly-concerned with aesthetic of tabs in XnView MP.

When I open a high-resolution image file from "recent files" I always notice this tab growth 'jump' due to XnView MP having to render the 16x16 thumbnail in the last slowest phase.
This is the case with high-resolution or detailed/complicated graphic PNG images. Issue might possibly be related to long filepath's too.
I reproduced this issue with these two examples: I believe to simply fix this, is that every tab that opens in MP should always open to the tab size of a ghost 16x16 thumb and then when the thumbnail is ready to load then it will load and this should be so for really high-resolution pictures (e.g. 6000x5000 pixels or higher etc).
OR
Another alternative (I think) would be to load a simple blank 16x16 transparent image (GIF) as a blank-placeholder of a thumb on a tab
This is an example of a simple basic 16x16 transparent GIF needed: I added grey borderline on 16x16 for effect but borderline of course is not essential...
.....It would be for ALL and EVERY tab opening cases.
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btw, I have always noticed this issue since early versions of XnView MP.
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XnView MP 0.39 -x86 -Windows 7 sp1