I'm certainly interested in this as the official version of slackwareaarch64 has been broken on the Pi for sometime (KDE doesn't work at all, XFCE is very buggy). Both KDE and XFCE work perfectly with the Sarpi kernel on slackwareaarch64 and on slarm64, so it is almost certainly a kernel issue causing the problem. Strange, as they used to work, if somewhat clunky, but with recent kernel releases (since changing to the VC4 driver?) either don't work at all, or are extremely buggy.
I've had a search for these Pi kernels, but can't find them to try out. Where have you hidden them?
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Pete