[LyX/master] Another go at fixing string metrics

[Jean-Marc Lasgouttes]

commit 5861df71721ddf823e5b01e0d9b8f4b97da58a97
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Sat Feb 17 16:55:22 2018 +0100

    Another go at fixing string metrics
    
    This time, it is the problems with \int in esint and cmex that we are
    trying to work around. This code is becoming too complicated to my
    taste.
---
 src/frontends/qt4/GuiFontMetrics.cpp |   32 ++++++++++++++++++++++----------
 1 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/src/frontends/qt4/GuiFontMetrics.cpp 
b/src/frontends/qt4/GuiFontMetrics.cpp
index b9eab85..c2d4b88 100644
--- a/src/frontends/qt4/GuiFontMetrics.cpp
+++ b/src/frontends/qt4/GuiFontMetrics.cpp
@@ -181,26 +181,38 @@ int GuiFontMetrics::width(docstring const & s) const
        if (strwidth_cache_.contains(s))
                return strwidth_cache_[s];
        PROFILE_CACHE_MISS(width);
-       /* For some reason QMetrics::width returns a wrong value with Qt5
-        * with some arabic text. OTOH, QTextLayout is broken for single
-        * characters with null width (like \not in mathed). Also, as a
-        * safety measure, always use QMetrics::width with our math fonts.
+       /* Several problems have to be taken into account:
+        * * QFontMetrics::width does not returns a wrong value with Qt5 with
+        *   some arabic text, since the glyph-shaping operations are not
+        *   done (documented in Qt5).
+        * * QTextLayout is broken for single characters with null width
+        *   (like \not in mathed).
+        * * While QTextLine::horizontalAdvance is the right thing to use
+     *   for text strings, it does not give a good result with some
+     *   characters like the \int (gyph 4) of esint.
+
+        * Also, as a safety measure, always use QFontMetrics::width with
+        * our math fonts.
        */
        int w = 0;
-       if (s.length() == 1
+       // is the string a single character from a math font ?
 #if QT_VERSION >= 0x040800
-           || font_.styleName() == "LyX"
+       bool const math_char = s.length() == 1 || font_.styleName() == "LyX";
+#else
+       bool const math_char = s.length() == 1;
 #endif
-           )
-               w = metrics_.width(toqstr(s));
-       else {
+       // keep value 0 for math chars with width 0
+       if (!math_char || metrics_.width(toqstr(s)) != 0) {
                QTextLayout tl;
                tl.setText(toqstr(s));
                tl.setFont(font_);
                tl.beginLayout();
                QTextLine line = tl.createLine();
                tl.endLayout();
-               w = iround(line.horizontalAdvance());
+               if (math_char)
+                       w = iround(line.naturalTextWidth());
+               else
+                       w = iround(line.horizontalAdvance());
        }
        strwidth_cache_.insert(s, w, s.size() * sizeof(char_type));
        return w;