Instead of calling reconfigure() on screen size changes, which deletes
and recreates the effect, we can just update the one texture that's mapped to
the screen size.

From the wayland commit.

What surprises me is that BlurManager recreates for Output changes.
That sounds like a bug in KWin (or an area which could be improved).

I had thought the same and wrote this (though wanted to have kwayland fixed first)

Logged in and out (my screen resizes on startup due to scaling)
Tested on my X machine by turning a screen off. Nothing broke, plasma
panels looked the same.

Note: Eike tried this patch last week and claimed kwin still crashed.
We don't have a backtrace so it could have been something unrelated,
applied wrong or that he was deliberately lying to wind me up.