Do not set layout for no-op paragraph break

When breaking paragraph in an empty top-level paragraph, nothing
happens on screen but yet there is an undo step because the layout is
reset to what it already was.

Avoid this case.

Fixes bug #10089.
(cherry picked from commit 2ad52918da)

src/Text.cpp

@@ -769,7 +769,8 @@ void Text::breakParagraph(Cursor & cur, bool inverse_logic)
 		else {
 			docstring const & lay = cur.paragraph().usePlainLayout()
 			    ? tclass.plainLayoutName() : tclass.defaultLayoutName();
-			setLayout(cur, lay);
+			if (lay != layout.name())
+				setLayout(cur, lay);
 		}
 		return;
 	}

status.22x

@@ -106,6 +106,8 @@ What's new
 
 - Fix the minimum size of the citation dialog on first time (bug 10019).
 
+- When Enter does nothing (empty paragraph), do not record undo (bug 10089).
+
 
 * INTERNALS