(struct window): New field optional_new_start.

src/window.h

@@ -112,6 +112,9 @@ struct window
     /* Non-nil means next redisplay must use the value of start
        set up for it in advance.  Set by scrolling commands.  */
     Lisp_Object force_start;
+    /* Non-nil means we have explicitly changed the value of start,
+       but that the next redisplay is not obliged to use the new value.  */
+    Lisp_Object optional_new_start;
     /* Number of columns display within the window is scrolled to the left.  */
     Lisp_Object hscroll;
     /* Number saying how recently window was selected */