[konsole] [Bug 451429] New: Fixed size scrollback overflowing moves scrolled up text

[Jonathan Marten]

https://bugs.kde.org/show_bug.cgi?id=451429

            Bug ID: 451429
           Summary: Fixed size scrollback overflowing moves scrolled up
                    text
           Product: konsole
           Version: master
          Platform: Other
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: history
          Assignee: konsole-devel at kde.org
          Reporter: jjm at keelhaul.me.uk
  Target Milestone: ---

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the discarding of outdated scrollback somehow seems to shift the displayed text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)

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