Avoid assertion violations when scrolling narrowed buffer

* src/window.c (window_scroll_pixel_based):
* src/xdisp.c (pos_visible_p): Don't allow simulated redisplay to
start outside the accessible portion of the buffer.  This avoids
assertion violations when some Lisp narrows the buffer to less
than the current window, and then attempts to scroll the buffer.

2 files changed, 6 insertions, 4 deletions

diff --git a/src/window.c b/src/window.c
index 374ef0f838f..753ebc16fbf 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4813,8 +4813,9 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, bool noerror)
   SET_TEXT_POS_FROM_MARKER (start, w->start);
   /* Scrolling a minibuffer window via scroll bar when the echo area
      shows long text sometimes resets the minibuffer contents behind
-     our backs.  */
-  if (CHARPOS (start) > ZV)
+     our backs.  Also, someone might narrow-to-region and immediately
+     call a scroll function.  */
+  if (CHARPOS (start) > ZV || CHARPOS (start) < BEGV)
     SET_TEXT_POS (start, BEGV, BEGV_BYTE);
 
   /* If PT is not visible in WINDOW, move back one half of
diff --git a/src/xdisp.c b/src/xdisp.c
index dc68cd48e5f..d1e884820b6 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -1317,8 +1317,9 @@ pos_visible_p (struct window *w, ptrdiff_t charpos, int *x, int *y,
   SET_TEXT_POS_FROM_MARKER (top, w->start);
   /* Scrolling a minibuffer window via scroll bar when the echo area
      shows long text sometimes resets the minibuffer contents behind
-     our backs.  */
-  if (CHARPOS (top) > ZV)
+     our backs.  Also, someone might narrow-to-region and immediately
+     call a scroll function.  */
+  if (CHARPOS (top) > ZV || CHARPOS (top) < BEGV)
     SET_TEXT_POS (top, BEGV, BEGV_BYTE);
 
   /* If the top of the window is after CHARPOS, the latter is surely