This one https://www.raspberrypi.com/products/raspberry-pi-touch-display/ seems like it would be a likely piece of test hardware. But the b2qt 6.5.7 pi 64-bit image does not use the right resolution by default on this screen. (With another 1920x1080 touchscreen that I have, it's ok.)

So far I haven't been able to troubleshoot further because of QTBUG-130031.

Setting

hdmi_group=1
hdmi_mode=3

in config.txt did not noticeably change anything. I wonder if that's no longer the right way to configure the mode, because of switching to kms.