Fix placement of limits with integral signs

Since be836909c5 the positioning of super- and subscripts
for symbol fonts has been broken because the metrics of the
font of the environment (rather than those of the symbol itself)
were used.

src/frontends/qt/GuiFontMetrics.cpp

@@ -226,8 +226,17 @@ int GuiFontMetrics::width(docstring const & s) const
      *   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.
+	 * The metrics of some of our math fonts (eg. esint) are such that
+	 * QTextLine::horizontalAdvance leads, more or less, in the middle
+	 * of a symbol. This is the horizontal position where a subscript
+	 * should be drawn, so that the superscript has to be moved rightward.
+	 * This is done when the kerning() method of the math insets returns
+	 * a positive value. The problem with this choice is that navigating
+	 * a formula becomes weird. For example, a selection extends only over
+	 * about half of the symbol. In order to avoid this, with our math
+	 * fonts we use QTextLine::naturalTextWidth, so that a superscript can
+	 * be drawn right after the symbol, and move the subscript leftward by
+	 * recording a negative value for the kerning.
 	*/
 	int w = 0;
 	// is the string a single character from a math font ?

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_);

src/mathed/InsetMathSymbol.cpp

@@ -68,14 +68,11 @@ Limits InsetMathSymbol::defaultLimits() const
 
 void InsetMathSymbol::metrics(MetricsInfo & mi, Dimension & dim) const
 {
-	mathedSymbolDim(mi.base, dim, sym_);
+	// set dim
+	// FIXME: this should depend on BufferView
+	// set negative kerning_ so that a subscript is moved leftward
+	kerning_ = -mathedSymbolDim(mi.base, dim, sym_);
 	if (sym_->draw != sym_->name) {
-		// set dim
-		// FIXME: this should depend on BufferView
-		// set kerning_
-		kerning_ = mathed_char_kerning(mi.base.font,
-		                               mathedSymbol(mi.base, sym_).back());
-
 		// align character vertically
 		// FIXME: this should depend on BufferView
 		h_ = 0;

src/mathed/MathSupport.cpp

@@ -680,9 +680,9 @@ docstring const &  mathedSymbol(MetricsBase & mb, latexkeys const * sym)
 }
 
 
-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
@@ -694,6 +694,7 @@ 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, mathedSymbol(mb, sym), dim);
+	return mathed_char_kerning(mb.font, mathedSymbol(mb, sym).back());
 }
 
 

src/mathed/MathSupport.h

@@ -57,7 +57,7 @@ int mathed_string_width(FontInfo const &, docstring const & s);
 
 docstring const & mathedSymbol(MetricsBase & mb, latexkeys const * sym);
 
-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);