Correctly implement rule 18a in appendix G of the TeXbook

* src/mathed/InsetMathScript.C
	(isAlphaSymbol): New. Return true if nucleus is a letter or a
	symbol whose type is mathord or mathalpha.
	(InsetMathScript::dy01): Apply rule 18a accounting for nucleus type.

	* src/mathed/MathData.C
	(MathArray::metrics): Account for the fact that some fonts (eg.
	mathcal) do not have a lower case x glyph.

	* src/mathed/InsetMathSymbol.[Ch]
	(InsetMathSymbol::isOrdAlpha): New. Return true if type of symbol
	is mathord or mathalpha.


git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@17518 a592a061-630c-0410-9148-cb99ea01b6c8

src/mathed/InsetMathScript.C

@@ -15,6 +15,7 @@
 #include "MathStream.h"
 #include "MathSupport.h"
 #include "InsetMathSymbol.h"
+#include "InsetMathFont.h"
 #include "dispatchresult.h"
 #include "cursor.h"
 #include "debug.h"
@@ -146,22 +147,48 @@ MathArray & InsetMathScript::nuc()
 }
 
 
+namespace {
+
+bool isAlphaSymbol(MathAtom const & at)
+{
+	if (at->asCharInset() ||
+			(at->asSymbolInset() &&
+			 at->asSymbolInset()->isOrdAlpha()))
+		return true;
+
+	if (at->asFontInset()) {
+		MathArray const & ar = at->asFontInset()->cell(0);
+		for (size_t i = 0; i < ar.size(); ++i) {
+			if (!(ar[i]->asCharInset() ||
+					(ar[i]->asSymbolInset() &&
+					 ar[i]->asSymbolInset()->isOrdAlpha())))
+				return false;
+		}
+		return true;
+	}
+	return false;
+}
+
+} // namespace anon
+
+
 int InsetMathScript::dy01(int asc, int des, int what) const
 {
 	int dasc = 0;
 	int slevel = 0;
+	bool isCharBox = nuc().size() ? isAlphaSymbol(nuc().back()) : false;
 	if (hasDown()) {
 		dasc = down().ascent();
 		slevel = nuc().slevel();
 		int ascdrop = dasc - slevel;
-		int desdrop = des + nuc().sshift();
+		int desdrop = isCharBox ? 0 : des + nuc().sshift();
 		int mindes = nuc().mindes();
 		des = max(desdrop, ascdrop);
 		des = max(mindes, des);
 	}
 	if (hasUp()) {
 		int minasc = nuc().minasc();
-		int ascdrop = asc - up().mindes();
+		int ascdrop = isCharBox ? 0 : asc - up().mindes();
 		int udes = up().descent();
 		asc = udes + nuc().sshift();
 		asc = max(ascdrop, asc);

src/mathed/InsetMathSymbol.C

@@ -116,6 +116,12 @@ bool InsetMathSymbol::isRelOp() const
 }
 
 
+bool InsetMathSymbol::isOrdAlpha() const
+{
+	return sym_->extra == "mathord" || sym_->extra == "mathalpha";
+}
+
+
 bool InsetMathSymbol::isScriptable() const
 {
 	return scriptable_;

src/mathed/InsetMathSymbol.h

@@ -42,6 +42,8 @@ public:
 
 	///
 	bool isRelOp() const;
+	///
+	bool isOrdAlpha() const;
 	/// do we take scripts?
 	bool isScriptable() const;
 	/// do we take \limits or \nolimits?

src/mathed/MathData.C

@@ -242,8 +242,11 @@ bool isInside(DocIterator const & it, MathArray const & ar,
 
 void MathArray::metrics(MetricsInfo & mi) const
 {
-	dim_ = theFontMetrics(mi.base.font).dimension('I');
-	int xascent = theFontMetrics(mi.base.font).dimension('x').ascent();
+	frontend::FontMetrics const & fm = theFontMetrics(mi.base.font);
+	dim_ = fm.dimension('I');
+	int xascent = fm.dimension('x').ascent();
+	if (xascent >= dim_.asc)
+		xascent = (2 * dim_.asc) / 3;
 	minasc_ = xascent;
 	mindes_ = (3 * xascent) / 4;
 	slevel_ = (4 * xascent) / 5;