[konsole] [Bug 379318] New: CSI S escape sequence (SU, scroll up) ignored if number of lines to scroll bigger than scrollable lines

[Moritz Bunkus]

https://bugs.kde.org/show_bug.cgi?id=379318

            Bug ID: 379318
           Summary: CSI S escape sequence (SU, scroll up) ignored if
                    number of lines to scroll bigger than scrollable lines
           Product: konsole
           Version: 17.04.0
          Platform: Archlinux Packages
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: NOR
         Component: emulation
          Assignee: konsole-devel at kde.org
          Reporter: moritz at bunkus.org
  Target Milestone: ---

Created attachment 105242
  --> https://bugs.kde.org/attachment.cgi?id=105242&action=edit
screenshot showing the corrupted output

Running konsole → tmux v2.4 I noticed screen corruption whenever I output
certain types of texts, e.g. the output of "certtool -i < some-cert.pem" which
dumps an X509 certificate in text form including an ASCII pictogram. I've
reported this to tmux[1] where you can see more details.

The tmux developers looked into it and were able to reproduce it with konsole,
but neither with xterm nor with gnome-terminal. According to [2] this is a
problem in konsole. What happens is that tmux sends a scroll up event with
quite a number of lines, in fact more lines that konsole is currently
displaying. In that case konsole ignores the whole scroll up command. Again
according to [2] this is a bug in konsole.

How to reproduce:

• download this[3] test file (this is the aforementioned output of "certtool")
• start tmux v2.4 with its default configuration, e.g. "tmux -Ltest
-f/dev/null"
• run "cat tmux-causes-bad-output.txt", maybe multiple times as the output
isn't consistent over multiple iterations, and observe the broken behavior

The author of tmux has a simple test to show off the problem:

• Resize konsole so that fewer than ten but more than five lines are shown
• Type a bit of text so that there's something to scroll
• Enter "printf '\033#8\033[10;15r\033[10S'" and observe that the content isn't
scrolled off properly (tries to scroll ten lines)
• Try "printf '\033#8\033[10;15r\033[5S'" instead and observe that the content
is scrolled off properly (tries to scroll five lines)

[1] https://github.com/tmux/tmux/issues/893
[2] https://github.com/tmux/tmux/issues/893#issuecomment-298041424
[3] https://www.bunkus.org/misc/tmux-causes-bad-output.txt

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