simon-git: putty (main): Simon Tatham

[Commits to Tartarus hosted VCS]

TL;DR:
  7a25599d Fix terminal redraw slowdown in presence of true colour.

Repository:     https://git.tartarus.org/simon/putty.git
On the web:     https://git.tartarus.org/?p=simon/putty.git
Branch updated: main
Committer:      Simon Tatham <anakin at pobox.com>
Date:           2021-07-15 20:19:36

commit 7a25599d8452c29bf32a5cd7de5968acb15732cd
web diff https://git.tartarus.org/?p=simon/putty.git;a=commitdiff;h=7a25599d8452c29bf32a5cd7de5968acb15732cd;hp=640e46a11254f553c9835b4e9a6f60578c6adc95
Author: Simon Tatham <anakin at pobox.com>
Date:   Thu Jul 15 20:02:55 2021 +0100

    Fix terminal redraw slowdown in presence of true colour.
    
    When do_paint breaks up a line of terminal text into contiguous runs
    of characters to treat the same, one of the criteria it uses is, 'Does
    this character even need redrawing? (or is it already displayed
    correctly from the previous redraw?)' When we encounter a character
    that matches its previous value, we end the previous run of
    characters, so that we can skip the one we've just encountered.
    
    That check was not taking account of the 'truecolour' field of the
    termchar it was checking. So it would sometimes falsely believe the
    character to be equivalent to its previously drawn value, even when in
    fact it was not, and hence insert a run break, anticipating that the
    previous character needed drawing and the current one did not.
    
    This didn't cause a _wrong_ redraw, because there's a separate loop
    further on which re-checks whether to actually draw things, which
    didn't make the same error. So the character that loop #1 thought
    didn't need a redraw, loop #2 knew _did_ need a redraw, and hence,
    everything did get redrawn.
    
    But by the time loop #2 is running, it's too late to change the run
    boundaries. So everything does get redrawn, but in much smaller chunks
    than it could have been. The net effect was that if the screen was
    filled with text displayed in true colour, and you changed it to the
    _same_ text in a different colour, then the whole terminal would be
    redrawn in one-character increments instead of the usual behaviour of
    folding together runs that can be drawn in one go.
    
    Thanks to Bradley Smith for debugging this very confusing issue!

 terminal.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)