Unfortunately, mesa 21.2.4-1 shows the same problem under plasma 5.23 in testing. The latter has finally migrated to testing with all packages yesterday. The combination works fine within my libvirt vm, as I think, it does not use hw accelleration there. However, on bare metal (see above) it stays unusable exactly as described. (OT: As plasma 5.23 depends on mesa >= 21.2.4 it still can't be used and package upgrades are even more difficult regarding the even larger mass of packes now to distiguish).
Chris
