Fix bug #8727: Text::breakParagraph() was always setting the default

layout, even if the paragraph in question requires plain layout.
(cherry picked from commit f8f1232465)

src/Text.cpp

@@ -695,8 +695,11 @@ void Text::breakParagraph(Cursor & cur, bool inverse_logic)
 	if (cur.lastpos() == 0 && !cpar.allowEmpty()) {
 		if (changeDepthAllowed(cur, DEC_DEPTH))
 			changeDepth(cur, DEC_DEPTH);
-		else 
-			setLayout(cur, tclass.defaultLayoutName());
+		else {
+			docstring const & lay = cur.paragraph().usePlainLayout()
+			    ? tclass.plainLayoutName() : tclass.defaultLayoutName();
+			setLayout(cur, lay);
+		}
 		return;
 	}
 

status.20x

@@ -89,6 +89,8 @@ What's new
 
 - Disable invalid actions in empty outliner that could trigger a crash
   (bug 8885).
+ 
+- Fix warning when setting layout box when Plain Layout is used (bug 8727).
 
 - Fix bug where searching for next change may leave an empty paragraph
   where cursor was (bug 3199).