i get 0.1mS , maybe my oscilloscope is off
Your scope is fine. All my quoted measurements are for my current configuration with 81 displays, so roughly double your measurement.

works as expected even when direction reversed midstream
It clearly does. Do you have a non visible are in the buffer? I can't see how it would matter but again, in my system it does weird things with what's outside the actual display.

The scrolling window was cool!