What would be useful here, is a graphical field in the wallpaper system settings to tweak fade time (and then you can enter 0 ms, for example).
IMO the way this conversation has gone so far, is a mistaken way to look at this problem. Instead of trying to fix the issue upstream, this “solution” includes hacky workarounds that only helps people who find this post and manage to reproduce it. The downsides however include:
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Fixes like these often do not last, and it might even break tomorrow, also considering LLMs were involved with things the user doesn’t quite understand.
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Apart from the individuals who find it, nobody else benefits from this on the whole. You’ve essentially “forked” (metaphorically) some part of KDE, applied the fix, but did not upstream the issue.
Solution: Feature request (and being as objective as you can, with detail)
Go to System Settings → Wallpaper. Then click the hamburger button → “Report a Bug in the Current Page”
You will be sent to bug report template on bugs.kde.org with the essential fields already filled in. Make sure to fill in any missing fields, and use the description template they provide.
This takes time and care. It might take a little longer than to write up a post like this. But it’s also the best way to have fixes and improvements benefit everyone (including yourself).
Also just keep it to reporting only, suggesting a technical solution is not needed, unless you want to fix bug upstream yourself, but you can link this forum post to give the developers some hints.