[LyX/master] Set a minimal height for math rows

[Jean-Marc Lasgouttes]

commit 6df8c42e59f7dbe00d585525d1ce7e91a42411ca
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Mon Apr 9 12:18:24 2018 +0200

    Set a minimal height for math rows
    
    The value used here is the x height. It would be nice to see what the
    TeXbook really says.
    
    This makes a big difference with \tilde{.}, for example.
---
 src/mathed/MathRow.cpp |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/mathed/MathRow.cpp b/src/mathed/MathRow.cpp
index 3459620..5610815 100644
--- a/src/mathed/MathRow.cpp
+++ b/src/mathed/MathRow.cpp
@@ -236,7 +236,10 @@ int MathRow::after(int i) const
 
 void MathRow::metrics(MetricsInfo & mi, Dimension & dim)
 {
-       dim.asc = 0;
+       frontend::FontMetrics const & fm = theFontMetrics(mi.base.font);
+       // FIXME: this is the minimal ascent seen empirically, check
+       // what the TeXbook says.
+       dim.asc = fm.ascent('x');
        dim.wid = 0;
        // In order to compute the dimension of macros and their
        // arguments, it is necessary to keep track of them.
@@ -284,7 +287,7 @@ void MathRow::metrics(MetricsInfo & mi, Dimension & dim)
                        }
                        break;
                case BOX:
-                       d = theFontMetrics(mi.base.font).dimension('I');
+                       d = fm.dimension('I');
                        if (e.color != Color_none) {
                                // allow for one pixel before/after the box.
                                d.wid += e.before + e.after + 2;