Do not error on missing ZWNJ (ligature break) characters.

Since TeXLive 2016, "fontspec" maps the ligature break command
\textcompwordmark to the ZWNJ character (U+200C).
This character is missing in many fonts (including the default: Latin
Modern) which leads to "Missing character" warnings in the XeTeX/LuaTeX
log file if a document using non-TeX fonts contains a ligature break.

LyX reports missing characters as error since fixing #9610.
In case of "invisible" characters, there is no data loss, in case of the
ZWNJ the functionality is kept: ligatures are prevented also if the ZWNJ
is missing in a font.

Therefore, a missing ZWNJ is now treated similar to missing characters
in "nullfont" (see [63f41711/lyxgit], bug #10394) and does not trigger
an error.

Fixes: #10727
(cherry picked from commit a40868510d)

src/LaTeX.cpp

@@ -911,11 +911,18 @@ int LaTeX::scanLogFile(TeXErrors & terr)
 				// Warning about missing glyph in selected font
 				// may be dataloss (bug 9610)
 				// but can be ignored for 'nullfont' (bug 10394).
-				retval |= LATEX_ERROR;
-				terr.insertError(0,
-						 from_local8bit("Missing glyphs!"),
-						 from_local8bit(token),
-						 child_name);
+				// as well as for ZERO WIDTH NON-JOINER (0x200C) which is
+				// missing in many fonts and output for ligature break (bug 10727).
+				// Since this error only occurs with utf8 output, we can safely assume
+				// that the log file is utf8-encoded
+				docstring const utoken = from_utf8(token);
+				if (!contains(utoken, 0x200C)) {
+					retval |= LATEX_ERROR;
+					terr.insertError(0,
+							 from_ascii("Missing glyphs!"),
+							 utoken,
+							 child_name);
+				}
 			}
 		}
 	}

status.22x

@@ -49,6 +49,9 @@ What's new
 
 - Catch "! Incomplete \if[x]" LaTeX error (bug 10666).
 
+- Ignore LaTeX error on missing ZWNJ (ligature break) characters
+  (bug 10727).
+
 - Fix PDF output of clipped graphics with recent graphics package (bug 7910).
 
 - Catch xdvipdfmx driver error that breaks XeTeX compilation (bug 10076).