On Thu, Jul 23, 2020 at 03:47:44PM +0200, Enrico Forestieri wrote:
> On Thu, Jul 23, 2020 at 03:24:13PM +0200, Jean-Marc Lasgouttes wrote:
> > Le 23/07/2020 à 15:13, Enrico Forestieri a écrit :
> > > On Thu, Jul 23, 2020 at 01:16:32PM +0200, Jean-Marc Lasgouttes wrote:
> > > 
> > > > Le 23/07/2020 à 12:22, Enrico Forestieri a écrit :
> > > > > Here is an alternative patch. Please check whether the change in
> > > > > InsetMathSymbol.cpp is compatible with the corresponding change
> > > > > at e8ee0100.
> > > > 
> > > > I think it is OK. What about the users of mathedSymbolDim in 
> > > > InsetMathFrac?
> > > > Is kerning irrelevant here?
> > 
> > This looks good for master and probably branch as far as I am concerned.
> 
> I am not really satisfied when navigating a formula with the cursor.
> The caret will be placed in the middle of a symbol instead of after it.
> Taking away the change to GuiFontMetrics.cpp makes navigation more
> pleasant but both scripts are placed far away the symbol.
> 
> I propose the attached patch that leaves unchanged GuiFontMetrics::width()
> and makes negative the kerning for a symbol, so that the subscript is
> moved to the left instead of moving the superscript to the right.

Patch pushed at 0b3e6916.

Richard, I attach here the corresponding patch for stable.

-- 
Enrico
diff --git a/src/frontends/qt4/GuiFontMetrics.cpp 
b/src/frontends/qt4/GuiFontMetrics.cpp
index 67fcb2f0e4..02c5054094 100644
--- a/src/frontends/qt4/GuiFontMetrics.cpp
+++ b/src/frontends/qt4/GuiFontMetrics.cpp
@@ -212,16 +212,19 @@ int GuiFontMetrics::width(docstring const & s) const
        /* 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.
+        * safety measure, always use QMetrics::boundingRect().width()
+        * with our math fonts.
        */
        int w = 0;
        if (s.length() == 1
 #if QT_VERSION >= 0x040800
            || font_.styleName() == "LyX"
 #endif
-           )
-               w = metrics_.width(toqstr(s));
-       else {
+           ) {
+               // keep value 0 for math chars with null width
+               if (metrics_.width(toqstr(s)) != 0)
+                       w = metrics_.boundingRect(toqstr(s)).width();
+       } else {
                QTextLayout tl;
                tl.setText(toqstr(s));
                tl.setFont(font_);
diff --git a/src/mathed/InsetMathChar.cpp b/src/mathed/InsetMathChar.cpp
index 1137c95e75..3a9d18c5c2 100644
--- a/src/mathed/InsetMathChar.cpp
+++ b/src/mathed/InsetMathChar.cpp
@@ -113,9 +113,7 @@ void InsetMathChar::metrics(MetricsInfo & mi, Dimension & 
dim) const
        if (isMathFont(f) && subst_) {
                // If the char has a substitute, draw the replacement symbol
                // instead, but only in math mode.
-               mathedSymbolDim(mi.base, dim, subst_);
-               kerning_ = mathed_char_kerning(mi.base.font, 
*subst_->draw.rbegin());
-               return;
+               kerning_ = mathedSymbolDim(mi.base, dim, subst_);
        } else if (!slanted(char_) && f == "mathnormal") {
                Changer dummy = mi.base.font.changeShape(UP_SHAPE);
                dim = theFontMetrics(mi.base.font).dimension(char_);
diff --git a/src/mathed/InsetMathSymbol.cpp b/src/mathed/InsetMathSymbol.cpp
index 92d2573560..106780e578 100644
--- a/src/mathed/InsetMathSymbol.cpp
+++ b/src/mathed/InsetMathSymbol.cpp
@@ -60,10 +60,8 @@ docstring InsetMathSymbol::name() const
 
 void InsetMathSymbol::metrics(MetricsInfo & mi, Dimension & dim) const
 {
-       // set dim
-       mathedSymbolDim(mi.base, dim, sym_);
-       // set kerning_
-       kerning_ = mathed_char_kerning(mi.base.font, *sym_->draw.rbegin());
+       // set dim and negative kerning_ to move a subscript leftward
+       kerning_ = -mathedSymbolDim(mi.base, dim, sym_);
        // correct height for broken cmex and wasy font
        if (sym_->inset == "cmex" || sym_->inset == "wasy") {
                h_ = 4 * dim.des / 5;
diff --git a/src/mathed/MathSupport.cpp b/src/mathed/MathSupport.cpp
index b9ea145c67..26d508f12c 100644
--- a/src/mathed/MathSupport.cpp
+++ b/src/mathed/MathSupport.cpp
@@ -673,9 +673,9 @@ void mathed_draw_deco(PainterInfo & pi, int x, int y, int 
w, int h,
 }
 
 
-void mathedSymbolDim(MetricsBase & mb, Dimension & dim, latexkeys const * sym)
+int mathedSymbolDim(MetricsBase & mb, Dimension & dim, latexkeys const * sym)
 {
-       LASSERT((bool)sym, return);
+       LASSERT((bool)sym, return 0);
        //lyxerr << "metrics: symbol: '" << sym->name
        //      << "' in font: '" << sym->inset
        //      << "' drawn as: '" << sym->draw
@@ -687,6 +687,7 @@ void mathedSymbolDim(MetricsBase & mb, Dimension & dim, 
latexkeys const * sym)
        std::string const font = italic_upcase_greek ? "cmm" : sym->inset;
        Changer dummy = mb.changeFontSet(font);
        mathed_string_dim(mb.font, sym->draw, dim);
+       return mathed_char_kerning(mb.font, sym->draw.back());
 }
 
 
diff --git a/src/mathed/MathSupport.h b/src/mathed/MathSupport.h
index d9301453dd..7c40baa0b4 100644
--- a/src/mathed/MathSupport.h
+++ b/src/mathed/MathSupport.h
@@ -55,7 +55,7 @@ void mathed_string_dim(FontInfo const & font,
 
 int mathed_string_width(FontInfo const &, docstring const & s);
 
-void mathedSymbolDim(MetricsBase & mb, Dimension & dim, latexkeys const * sym);
+int mathedSymbolDim(MetricsBase & mb, Dimension & dim, latexkeys const * sym);
 
 void mathedSymbolDraw(PainterInfo & pi, int x, int y, latexkeys const * sym);
 
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