New input parser based on http://vt100.net/emu/dec_ansi_parser.

window.c

@@ -623,6 +623,81 @@ window_pane_resize(struct window_pane *wp, u_int sx, u_int sy)
 		fatal("ioctl failed");
 }
 
+/*
+ * Enter alternative screen mode. A copy of the visible screen is saved and the
+ * history is not updated
+ */
+void
+window_pane_alternate_on(struct window_pane *wp, struct grid_cell *gc)
+{
+	struct screen	*s = &wp->base;
+	u_int		 sx, sy;
+
+	if (wp->saved_grid != NULL)
+		return;
+	if (!options_get_number(&wp->window->options, "alternate-screen"))
+		return;
+	sx = screen_size_x(s);
+	sy = screen_size_y(s);
+
+	wp->saved_grid = grid_create(sx, sy, 0);
+	grid_duplicate_lines(wp->saved_grid, 0, s->grid, screen_hsize(s), sy);
+	wp->saved_cx = s->cx;
+	wp->saved_cy = s->cy;
+	memcpy(&wp->saved_cell, gc, sizeof wp->saved_cell);
+
+	grid_view_clear(s->grid, 0, 0, sx, sy);
+
+	wp->base.grid->flags &= ~GRID_HISTORY;
+
+	wp->flags |= PANE_REDRAW;
+}
+
+/* Exit alternate screen mode and restore the copied grid. */
+void
+window_pane_alternate_off(struct window_pane *wp, struct grid_cell *gc)
+{
+	struct screen	*s = &wp->base;
+	u_int		 sx, sy;
+
+	if (wp->saved_grid == NULL)
+		return;
+	if (!options_get_number(&wp->window->options, "alternate-screen"))
+		return;
+	sx = screen_size_x(s);
+	sy = screen_size_y(s);
+
+	/*
+	 * If the current size is bigger, temporarily resize to the old size
+	 * before copying back.
+	 */
+	if (sy > wp->saved_grid->sy)
+		screen_resize(s, sx, wp->saved_grid->sy);
+
+	/* Restore the grid, cursor position and cell. */
+	grid_duplicate_lines(s->grid, screen_hsize(s), wp->saved_grid, 0, sy);
+	s->cx = wp->saved_cx;
+	if (s->cx > screen_size_x(s) - 1)
+		s->cx = screen_size_x(s) - 1;
+	s->cy = wp->saved_cy;
+	if (s->cy > screen_size_y(s) - 1)
+		s->cy = screen_size_y(s) - 1;
+	memcpy(gc, &wp->saved_cell, sizeof *gc);
+
+	/*
+	 * Turn history back on (so resize can use it) and then resize back to
+	 * the current size.
+	 */
+	wp->base.grid->flags |= GRID_HISTORY;
+	if (sy > wp->saved_grid->sy)
+		screen_resize(s, sx, sy);
+
+	grid_destroy(wp->saved_grid);
+	wp->saved_grid = NULL;
+
+	wp->flags |= PANE_REDRAW;
+}
+
 int
 window_pane_set_mode(struct window_pane *wp, const struct window_mode *mode)
 {