This is excellent; thank you and happy to help diagnose where I can. Bit rushed tonight but think I tried most items and I hope some of this is helpful. Let me know if I missed anything important or other things to try and over next couple of evenings I should have a bit of time.
(Also, thank you for the clarification on TN and Preview sizes. My bad for assuming. Explains what I saw on the larger previews.)
In order I tried things:
A) try [preview_load_size] @ 256 (not 0 or 1024 default)
• seemed to help some for preview / but not all; bit anecdotal
• TN creation still hit and miss
• back to 1024 or 0 and prior behavior returned (no-prev 1st visit, yes preview on 2nd row visit)
B) try [thumbnail_load_size] @ 256
• seemed to successfully create TNs for ALL -further-down- images, as scrolled, earlier ones already in view stayed at missing; did not recreate those TNs
• when both TNs and Preview set at 256, seems ALL cases work (though previews are too small to be too useful of course)
• for ones where TNs made through manual 256 setting, then it seems virtually all previews work, even at 1024 or 2048 size
• With quick tests, the workaround seemed to resolve: JPG, PNG, TIF, GIF, / but NOT: SVG, BMP, PPT ... not sure; but seems maybe since different TN handlers
**so setting TNs to 128 or 256 as override seems an excellent stop-gap for now** - gets about 3/4 of my cases of interest to then work
C) try disable [shell_item_image_factory]
• with disabled, TNs hit and miss (same as before troubleshooting)
• Previews now ONLY worked at all for images that also succeeded at making TNs; even {visit-then-return} did NOT provide previews. Seems interesting.
D) try different TN handlers or disable ones
• hrm,... Google DriveFS has handler for: BMP, PNG / tried disabling but same behaviors for PNG (not enough BMPs to easily test)
• MOST seem from 2 MS built-in O/S ones
E) I tried making a log file - first bit has "the issue", many TNs failed to load. Changed settings at 19:05 to have TNs at 128 size. Then did search into different results to confirm the earlier mentioned findings. Hope it helps.
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(Side note - super clean & well done Advanced settings, with highlighting changed & the search. Very satisfying.)