[LyX/master] brkstr2str_pos: avoid case where length is negative

[Jean-Marc Lasgouttes]

commit ec3a44fef59129ba4492ddf30de81fa329e631d5
Author: Jean-Marc Lasgouttes <lasgouttes at lyx.org>
Date:   Sun Jan 23 19:34:11 2022 +0100

    brkstr2str_pos: avoid case where length is negative
    
    The string that Qt sees is not the original string, since we add
    zero-width nonbreaking spaces around it, plus an override character to
    force rendering direction. This means that translating positions
    between these two representation can be a bit tricky.
    
    We use a max(foo, 0) here to make sure that our intermediate
    computation is postive.
    
    Fixes part of bug #12452.
---
 src/frontends/qt/GuiFontMetrics.cpp |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/src/frontends/qt/GuiFontMetrics.cpp b/src/frontends/qt/GuiFontMetrics.cpp
index d827f2f..e7f61c6 100644
--- a/src/frontends/qt/GuiFontMetrics.cpp
+++ b/src/frontends/qt/GuiFontMetrics.cpp
@@ -535,7 +535,7 @@ docstring::size_type brkstr2str_pos(QString brkstr, docstring const & str, int p
 	 */
 	// The variable `brkStrOffset' is here to account for the extra leading characters.
 	// The ending character zerow_nbsp has to be ignored if the line is complete.
-	int const qlen = pos - brkStrOffset - (pos == brkstr.length());
+	int const qlen = max(pos - brkStrOffset - (pos == brkstr.length()), 0);
 #if QT_VERSION < 0x040801 || QT_VERSION >= 0x050100
 	auto const len = qstring_to_ucs4(brkstr.mid(brkStrOffset, qlen)).length();
 	// Avoid warning