Glitchy behaviour in Firefox and other places

I just did a -current upgrade from last 2 weeks and have inherited a number of strange glitches. (See attached screenshot).

They're most noticable in Firefox in particular places on particular sites, and particularly when highlighting text.

But not only in Firefox. I noticed it also in sddm and on some window decorations in other places.

I am on Intel graphics, however the driver has not changed in this period it seems, nor has my X11 config. I use xfce for my desktop.

Any tips welcome :-)

Andypoo.
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