SUMMARY
Normally, when the terminal is scrolled backwards to view previous output, the view should then remain stationary regardless of what is being printed by the shell process.  However, if the fixed size (lines) of scrollback is exceeded, the discarding of outdated scrollback somehow seems to shift the displayed text in the view.  This is very disconcerting when scrolling back to view, e.g. previous compilation output, while more is being generated.

The effect is difficult to describe, but hopefully the following test cases will reproduce it.

STEPS TO REPRODUCE
1.  Start a new Konsole window with a specified fixed scrollback size - here set to 10,000 lines.
2.  Run a command which prints periodic output to the terminal:
     $ perl -e 'for (1..1000) { print $_,"\n"; sleep 1; }'
3.  Let the command run to fill a full page and start scrolling up, then scroll backwards using the scroll bar or mouse wheel.  Just a few lines will do.
4.  Observe that the scrolled back output remains stationary, although the scroll bar shrinks as more output is produced.
5.  Interrupt the previous command, then output enough lines to fill and overflow the scrollback history:
     $ perl -e 'for (1..20000) { print $_,"\n"; }'
6.  Start the periodic output again:
     $ perl -e 'for (1..1000) { print $_,"\n"; sleep 1; }'
7.  Scroll backwards again.  This time the scrolled back output will jump upwards by 5 lines every 5 seconds, for as long as more output is produced.  The scroll bar remains stationary.

OBSERVED RESULT
In step 7, scrolled back output moves as more output is produced.

EXPECTED RESULT
The scrolled back output should remain stationary.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.24.80
KDE Frameworks Version: 5.92.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
Konsole version: Git master (56abf298)