emulation: Disregard bce only when autowrapping to the new line

Apply bce (background color erase) on a line that newly appears due to an
explicit escape sequence, but not when the text overflows to the next line.

This improves the fix from commit 18171bc
and fixes the le editor.

https://bugzilla.gnome.org/show_bug.cgi?id=754596#c15

src/vte.cc

@@ -2773,7 +2773,7 @@ VteTerminalPrivate::cleanup_fragments(long start,
 
 /* Cursor down, with scrolling. */
 void
-VteTerminalPrivate::cursor_down()
+VteTerminalPrivate::cursor_down(bool explicit_sequence)
 {
 	long start, end;
 
@@ -2822,13 +2822,14 @@ VteTerminalPrivate::cursor_down()
 			update_insert_delta();
 		}
 
-		/* Match xterm and fill the new row when scrolling. */
-#if 0           /* Disable for now: see bug 754596. */
-                if (m_fill_defaults.attr.back != VTE_DEFAULT_BG) {
+                /* Handle bce (background color erase), however, diverge from xterm:
+                 * only fill the new row with the background color if scrolling
+                 * happens due to an explicit escape sequence, not due to autowrapping.
+                 * See bug 754596 for details. */
+                if (explicit_sequence && m_fill_defaults.attr.back != VTE_DEFAULT_BG) {
 			VteRowData *rowdata = ensure_row();
                         _vte_row_data_fill (rowdata, &m_fill_defaults, m_column_count);
 		}
-#endif
 	} else {
 		/* Otherwise, just move the cursor down. */
                 m_screen->cursor.row++;
@@ -2965,7 +2966,7 @@ VteTerminalPrivate::insert_char(gunichar c,
 			/* Mark this line as soft-wrapped. */
 			row = ensure_row();
 			row->attr.soft_wrapped = 1;
-			cursor_down();
+                        cursor_down(false);
 		} else {
 			/* Don't wrap, stay at the rightmost column. */
                         col = m_screen->cursor.col =

src/vteinternal.hh

@@ -560,7 +560,7 @@ public:
         void cleanup_fragments(long start,
                                long end);
 
-        void cursor_down();
+        void cursor_down(bool explicit_sequence);
         void drop_scrollback();
 
         void restore_cursor(VteScreen *screen__);

src/vteseq.cc

@@ -1661,7 +1661,7 @@ static void
 vte_sequence_handler_next_line (VteTerminalPrivate *that, GValueArray *params)
 {
         that->set_cursor_column(0);
-	that->cursor_down();
+        that->cursor_down(true);
 }
 
 /* No-op. */
@@ -1818,7 +1818,7 @@ vte_sequence_handler_line_feed (VteTerminalPrivate *that, GValueArray *params)
 {
         that->ensure_cursor_is_onscreen();
 
-	that->cursor_down();
+        that->cursor_down(true);
 }
 
 /* Cursor up 1 line, with scrolling. */