[LyX/2.3.x] Revert "Remove 'premature optimization' that proved buggy"

[Jean-Marc Lasgouttes]

commit f27d2cfa30aecd39afcac8a5b6532f9cf348aed6
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Sat Jan 27 12:03:09 2018 +0100

    Revert "Remove 'premature optimization' that proved buggy"
    
    This reverts commit 894ae19224d0537c0c9ebd9ad45e547279769bd8.
    
    Fixes bug 10996.
---
 src/TextMetrics.cpp |   12 ++++++++++--
 1 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/src/TextMetrics.cpp b/src/TextMetrics.cpp
index 6205144..6c5a322 100644
--- a/src/TextMetrics.cpp
+++ b/src/TextMetrics.cpp
@@ -834,6 +834,11 @@ bool TextMetrics::breakRow(Row & row, int const 
right_margin) const
        // the width available for the row.
        int const width = max_width_ - row.right_margin;
 
+       if (pos >= end || row.width() > width) {
+               row.endpos(end);
+               return need_new_row;
+       }
+
 #if 0
        //FIXME: As long as leftMargin() is not correctly implemented for
        // MARGIN_RIGHT_ADDRESS_BOX, we should also not do this here.
@@ -852,7 +857,10 @@ bool TextMetrics::breakRow(Row & row, int const 
right_margin) const
        // or the end of the par, then build a representation of the row.
        pos_type i = pos;
        FontIterator fi = FontIterator(*this, par, row.pit(), pos);
-       while (i < end && row.width() <= width) {
+       do {
+               // this can happen for an empty row after a newline
+               if (i >= end)
+                       break;
                char_type c = par.getChar(i);
                // The most special cases are handled first.
                if (par.isInset(i)) {
@@ -934,7 +942,7 @@ bool TextMetrics::breakRow(Row & row, int const 
right_margin) const
 
                ++i;
                ++fi;
-       }
+       } while (i < end && row.width() <= width);
        row.finalizeLast();
        row.endpos(i);