master d165b5a: New variable `redisplay_adhoc_scroll_in_resize_mini_windows`

[Stefan Monnier]

branch: master
commit d165b5a46b2a84c637a80200ad6bcf164bbfa77b
Author: Stefan Monnier <monnier@iro.umontreal.ca>
Commit: Stefan Monnier <monnier@iro.umontreal.ca>

    New variable `redisplay_adhoc_scroll_in_resize_mini_windows`
    
    * src/xdisp.c (syms_of_xdisp): Define it.
    (resize_mini_window): Obey it.
---
 etc/NEWS    |  7 +++++++
 src/xdisp.c | 58 ++++++++++++++++++++++++++++++++++++----------------------
 2 files changed, 43 insertions(+), 22 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index 901a432..5142095 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -88,6 +88,13 @@ useful on systems such as FreeBSD which ships only with 
"etc/termcap".
 ** Minibuffer scrolling is now conservative by default.
 This is controlled by the new variable 'scroll-minibuffer-conservatively'.
 
+In addition, there is a new variable
+`redisplay-adhoc-scroll-in-resize-mini-windows` to disable the
+ad-hoc auto-scrolling when resizing minibuffer windows.  It has been
+found that its heuristic can be counter productive in some corner
+cases, tho the cure may be worse than the disease.  This said, the
+effect should be negligible in the vast majority of cases anyway.
+
 +++
 ** Improved handling of minibuffers on switching frames.
 By default, when you switch to another frame, an active minibuffer now
diff --git a/src/xdisp.c b/src/xdisp.c
index 689b87d..96dd4fa 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -11751,9 +11751,10 @@ resize_mini_window (struct window *w, bool exact_p)
     return false;
 
   /* By default, start display at the beginning.  */
-  set_marker_both (w->start, w->contents,
-                  BUF_BEGV (XBUFFER (w->contents)),
-                  BUF_BEGV_BYTE (XBUFFER (w->contents)));
+  if (redisplay_adhoc_scroll_in_resize_mini_windows)
+    set_marker_both (w->start, w->contents,
+                    BUF_BEGV (XBUFFER (w->contents)),
+                    BUF_BEGV_BYTE (XBUFFER (w->contents)));
 
   /* Nil means don't try to resize.  */
   if ((NILP (Vresize_mini_windows)
@@ -11812,27 +11813,32 @@ resize_mini_window (struct window *w, bool exact_p)
       if (height > max_height)
        {
          height = (max_height / unit) * unit;
-         init_iterator (&it, w, ZV, ZV_BYTE, NULL, DEFAULT_FACE_ID);
-         move_it_vertically_backward (&it, height - unit);
-         /* The following move is usually a no-op when the stuff
-            displayed in the mini-window comes entirely from buffer
-            text, but it is needed when some of it comes from overlay
-            strings, especially when there's an after-string at ZV.
-            This happens with some completion packages, like
-            icomplete, ido-vertical, etc.  With those packages, if we
-            don't force w->start to be at the beginning of a screen
-            line, important parts of the stuff in the mini-window,
-            such as user prompt, will be hidden from view.  */
-         move_it_by_lines (&it, 0);
-         start = it.current.pos;
-         /* Prevent redisplay_window from recentering, and thus from
-            overriding the window-start point we computed here.  */
-         w->start_at_line_beg = false;
+         if (redisplay_adhoc_scroll_in_resize_mini_windows)
+           {
+             init_iterator (&it, w, ZV, ZV_BYTE, NULL, DEFAULT_FACE_ID);
+             move_it_vertically_backward (&it, height - unit);
+              /* The following move is usually a no-op when the stuff
+                 displayed in the mini-window comes entirely from buffer
+                 text, but it is needed when some of it comes from overlay
+                 strings, especially when there's an after-string at ZV.
+                 This happens with some completion packages, like
+                 icomplete, ido-vertical, etc.  With those packages, if we
+                 don't force w->start to be at the beginning of a screen
+                 line, important parts of the stuff in the mini-window,
+                 such as user prompt, will be hidden from view.  */
+              move_it_by_lines (&it, 0);
+              start = it.current.pos;
+              /* Prevent redisplay_window from recentering, and thus from
+                 overriding the window-start point we computed here.  */
+              w->start_at_line_beg = false;
+              SET_MARKER_FROM_TEXT_POS (w->start, start);
+            }
        }
       else
-       SET_TEXT_POS (start, BEGV, BEGV_BYTE);
-
-      SET_MARKER_FROM_TEXT_POS (w->start, start);
+       {
+         SET_TEXT_POS (start, BEGV, BEGV_BYTE);
+          SET_MARKER_FROM_TEXT_POS (w->start, start);
+        }
 
       if (EQ (Vresize_mini_windows, Qgrow_only))
        {
@@ -35502,6 +35508,14 @@ The initial frame is not displayed anywhere, so 
skipping it is
 best except in special circumstances such as running redisplay tests
 in batch mode.   */);
   redisplay_skip_initial_frame = true;
+
+  DEFVAR_BOOL ("redisplay-adhoc-scroll-in-resize-mini-windows",
+               redisplay_adhoc_scroll_in_resize_mini_windows,
+    doc: /* If nil always use normal scrolling in minibuffer windows.
+Otherwise, use custom-tailored code after resizing minibuffer windows to try
+and display the most important part of the minibuffer.   */);
+  /* See bug#43519 for some discussion around this.  */
+  redisplay_adhoc_scroll_in_resize_mini_windows = true;
 }