[LyX/master] Allow breaking a row element at a leading/trailing space

[Jean-Marc Lasgouttes]

commit 31471496fd53b20b684173da56ba59d0da7f90fc
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Tue Jan 19 12:02:43 2016 +0100

    Allow breaking a row element at a leading/trailing space
    
    When an inset is separated from the adjacent string by a space, it is
    reasonable to be able to break the string after the space.
    Unfortunately, QTextLayout does not do that.
    
    This patch reverts the workaround inserted in 71378268 and replaces it
    with a different trick: the string is enlosed between a pair of
    zero-width non breaking space characters, so that the leading/trailing
    spaces are now normal spaces, where QTextLayout will agree to break
    the string.
    
        Fixes bug #9921 for good.

diff --git a/src/Row.cpp b/src/Row.cpp
index 2bfe55a..6e51fb1 100644
--- a/src/Row.cpp
+++ b/src/Row.cpp
@@ -129,17 +129,6 @@ bool Row::Element::breakAt(int w, bool force)
                return true;
        }
 
-       // Qt will not break at a leading space, and we need that sometimes, see
-       //   http://www.lyx.org/trac/ticket/9921.
-       // It would be nice to fix this properly, but for now do it by hand.
-       // FIXME: figure out what to do for RtL text.
-       if (!isRTL() && !str.empty() && str[0] == ' ') {
-               dim.wid = 0;
-               str = ' ';
-               endpos = pos + 1;
-               return true;
-       }
-
        return false;
 }
 
diff --git a/src/frontends/qt4/GuiFontMetrics.cpp 
b/src/frontends/qt4/GuiFontMetrics.cpp
index c58be2a..fdb8c73 100644
--- a/src/frontends/qt4/GuiFontMetrics.cpp
+++ b/src/frontends/qt4/GuiFontMetrics.cpp
@@ -219,7 +219,16 @@ bool GuiFontMetrics::breakAt(docstring & s, int & x, bool 
const rtl, bool const
        if (s.empty())
                return false;
        QTextLayout tl;
-       tl.setText(toqstr(s));
+       /* Qt will not break at a leading or trailing space, and we need
+        * that sometimes, see http://www.lyx.org/trac/ticket/9921.
+        *
+        * To work around the problem, we enclose the string between
+        * zero-width characters so that the QTextLayout algorithm will
+        * agree to break the text at these extremal spaces.
+       */
+       // Unicode character ZERO WIDTH NO-BREAK SPACE
+       QChar const zerow_nbsp(0xfeff);
+       tl.setText(zerow_nbsp + toqstr(s) + zerow_nbsp);
        tl.setFont(font_);
        // Note that both setFlags and the enums are undocumented
        tl.setFlags(rtl ? Qt::TextForceRightToLeft : Qt::TextForceLeftToRight);
@@ -234,7 +243,8 @@ bool GuiFontMetrics::breakAt(docstring & s, int & x, bool 
const rtl, bool const
        if (int(line.naturalTextWidth()) > x)
                return false;
        x = int(line.naturalTextWidth());
-       s = s.substr(0, line.textLength());
+       // The -1 is here to account for the leading zerow_nbsp.
+       s = s.substr(0, line.textLength() - 1);
        return true;
 }