Handle properly unicode paragraph/line break

They are shown on screen by arrow or pilcrow symbol and cause a line break.

They are still not handled in LaTeX output, though.
(cherry picked from commit 6dfbab3124)

src/Paragraph.cpp

@@ -3153,6 +3153,11 @@ bool Paragraph::isHfill(pos_type pos) const
 
 bool Paragraph::isNewline(pos_type pos) const
 {
+	// U+2028 LINE SEPARATOR
+	// U+2029 PARAGRAPH SEPARATOR
+	char_type const c = d->text_[pos];
+	if (c == 0x2028 || c == 0x2029)
+		return true;
 	Inset const * inset = getInset(pos);
 	return inset && inset->lyxCode() == NEWLINE_CODE;
 }

src/TextMetrics.cpp

@@ -851,7 +851,23 @@ void TextMetrics::breakRow(Row & row, int const right_margin, pit_type const pit
 		} else if (c == '\t')
 			row.addSpace(i, theFontMetrics(*fi).width(from_ascii("    ")),
 				     *fi, par.lookupChange(i));
-		else {
+		else if (c == 0x2028 || c == 0x2029) {
+			/**
+			 * U+2028 LINE SEPARATOR
+			 * U+2029 PARAGRAPH SEPARATOR
+
+			 * These are special unicode characters that break
+			 * lines/pragraphs. Not handling them lead to trouble wrt
+			 * Qt QTextLayout formatting. We add a visible character
+			 * on screen so that the user can see that something is
+			 * happening.
+			*/
+			row.finalizeLast();
+			// ⤶ U+2936 ARROW POINTING DOWNWARDS THEN CURVING LEFTWARDS
+			// ¶ U+00B6 PILCROW SIGN
+			char_type const screen_char = (c == 0x2028) ? 0x2936 : 0x00B6;
+			row.add(i, screen_char, *fi, par.lookupChange(i));
+		} else {
 			// FIXME: please someone fix the Hebrew/Arabic parenthesis mess!
 			// see also Paragraph::getUChar.
 			if (fi->language()->lang() == "hebrew") {
@@ -911,6 +927,7 @@ void TextMetrics::breakRow(Row & row, int const right_margin, pit_type const pit
 		BufferParams const & bparams
 			= text_->inset().buffer().params();
 		f.setLanguage(par.getParLanguage(bparams));
+		// ¶ U+00B6 PILCROW SIGN
 		row.addVirtual(end, docstring(1, char_type(0x00B6)), f, Change());
 	}
 

status.22x

@@ -161,6 +161,8 @@ What's new
 - Fix cursor movement when the document contains high-plane Unicode
   characters (bug 10443).
 
+- Fix crash when the document contains Unicode line-breaking characters.
+
 - Allow using colors supported by xcolor inside mathed (bug 10417).
 
 - Change description of "frame of button" color to "button frame" (bug