[LyX/master] Fix computation of string width when using a QTextLayout

[Jean-Marc Lasgouttes]

commit c874641e95b763a6d4691fb12fba893580f3018a
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Mon Aug 28 12:05:35 2017 +0200

    Fix computation of string width when using a QTextLayout
    
    It was not a good idea to rely on QTextLine::naturalTextWidth() to
    compute a string width. The correct method is horizontalAdvance().
    
    Also round the value to the nearest pixel, since this is what
    QFontMetrics::width() does.
    
    Fixes bug #10700 (and maybe others).
---
 src/frontends/qt4/GuiFontMetrics.cpp |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/src/frontends/qt4/GuiFontMetrics.cpp 
b/src/frontends/qt4/GuiFontMetrics.cpp
index 1c00372..b9eab85 100644
--- a/src/frontends/qt4/GuiFontMetrics.cpp
+++ b/src/frontends/qt4/GuiFontMetrics.cpp
@@ -19,6 +19,7 @@
 
 #include "support/convert.h"
 #include "support/lassert.h"
+#include "support/lyxlib.h"
 
 #define DISABLE_PMPROF
 #include "support/pmprof.h"
@@ -199,7 +200,7 @@ int GuiFontMetrics::width(docstring const & s) const
                tl.beginLayout();
                QTextLine line = tl.createLine();
                tl.endLayout();
-               w = int(line.naturalTextWidth());
+               w = iround(line.horizontalAdvance());
        }
        strwidth_cache_.insert(s, w, s.size() * sizeof(char_type));
        return w;
@@ -386,7 +387,8 @@ GuiFontMetrics::breakAt_helper(docstring const & s, int 
const x,
        line.setLineWidth(x);
        tl.createLine();
        tl.endLayout();
-       if ((force && line.textLength() == offset) || 
int(line.naturalTextWidth()) > x)
+       int const line_wid = iround(line.horizontalAdvance());
+       if ((force && line.textLength() == offset) || line_wid > x)
                return {-1, -1};
        /* Since QString is UTF-16 and docstring is UCS-4, the offsets may
         * not be the same when there are high-plan unicode characters
@@ -411,8 +413,7 @@ GuiFontMetrics::breakAt_helper(docstring const & s, int 
const x,
                --len;
        LASSERT(len > 0 || qlen == 0, /**/);
 #endif
-       // The -1 is here to account for the leading zerow_nbsp.
-       return {len, int(line.naturalTextWidth())};
+       return {len, line_wid};
 }