bug#43572: Feature request: make it possible to choose whether the first lines of the minibuffer should be displayed instead of the last ones

[Gregory Heytings via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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>
> Really?  Despite the `-Q`?  Is it using a proportional font?
> Oh, wait, is it because you're running that Emacs session in a tty rather than in a GUI?
>

Yes, yes, no, no ;-)

>
> How many columns wide is the frame?
>

167, Sir! ;-)

>> No, if you look at the patch its value is reset to nil whenever the 
>> minibuffer is entered.
>
> That still leaves the non-minibuffer uses of the mini window (i.e. the 
> echo area).
>
> And of course, that will also result in the use of an incorrect value 
> when you exit a *nested* minibuffer (unless both minibuffers were using 
> the same value).
>

Thank you very much for the pointers, indeed I completely forgot about 
these cases.  See the attached corrected patch, in which they are dealt 
with.  I believe start_display_at_beginning_of_minibuffer how has the 
value it should have whenever resize_mini_window() is called.

>>> I don't have a patch to suggest, but I think ideally, I'd want clients 
>>> like icomplete to tell the redisplay either something like "please 
>>> display as much as possible of *this* chunk of text" or maybe "feel 
>>> free not to display all of this overlay, it's not super important".
>>
>> Yes, that's the point.  The patch I proposed tells redisplay "please 
>> display as much as possible of the text between BOB and point", when 
>> the current behavior tells redisplay "please display as much as 
>> possible of the text between point and EOB".  Something more 
>> fine-grained would of course be possible, but I don't think it is a 
>> good idea to implement a feature when there is no clear need for it.
>
> I think what I was getting at is that this "request" should come from 
> the minibuffer's text rather than from a variable.
>

If this were possible, it would be even better indeed.  An "importance" 
text property, which would inform redisplay (?) of the relative importance 
of the parts of the buffer.  But implementing this is several orders of 
magnitude harder than implementing my proposal.

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diff --git a/src/minibuf.c b/src/minibuf.c
index f957b2ae17..691a8c4f9b 100644
--- a/src/minibuf.c
+++ b/src/minibuf.c
@@ -472,6 +472,8 @@ read_minibuf (Lisp_Object map, Lisp_Object initial, Lisp_Object prompt,
     = Fcons (Voverriding_local_map,
 	     Fcons (minibuf_window,
 		    minibuf_save_list));
+  minibuf_save_list
+    = Fcons (Vstart_display_at_beginning_of_minibuffer, minibuf_save_list);
   minibuf_save_list
     = Fcons (minibuf_prompt,
 	     Fcons (make_fixnum (minibuf_prompt_width),
@@ -494,6 +496,7 @@ read_minibuf (Lisp_Object map, Lisp_Object initial, Lisp_Object prompt,
 
   /* Now that we can restore all those variables, start changing them.  */
 
+  Vstart_display_at_beginning_of_minibuffer = Qnil;
   minibuf_prompt_width = 0;
   minibuf_prompt = Fcopy_sequence (prompt);
   Vminibuffer_history_position = histpos;
@@ -802,6 +805,8 @@ read_minibuf_unwind (void)
   minibuf_save_list = Fcdr (minibuf_save_list);
   Vminibuffer_history_variable = Fcar (minibuf_save_list);
   minibuf_save_list = Fcdr (minibuf_save_list);
+  Vstart_display_at_beginning_of_minibuffer = Fcar (minibuf_save_list);
+  minibuf_save_list = Fcdr (minibuf_save_list);
   Voverriding_local_map = Fcar (minibuf_save_list);
   minibuf_save_list = Fcdr (minibuf_save_list);
 #if 0
@@ -2097,6 +2102,15 @@ syms_of_minibuf (void)
 uses to hide passwords.  */);
   Vread_hide_char = Qnil;
 
+  DEFVAR_LISP ("start-display-at-beginning-of-minibuffer", Vstart_display_at_beginning_of_minibuffer,
+	       doc: /* Whether to preferably display the beginning of the minibuffer.
+When the mini-window is not large enough to display the complete minibuffer,
+the default behavior is to display the last part of the minibuffer and to
+hide its first part.  This variable reverses that behavior.  This variable
+is reset before and after each read from the minibuffer, and must be set in
+`minibuffer-setup-hook'.  */);
+  Vstart_display_at_beginning_of_minibuffer = Qnil;
+
   defsubr (&Sactive_minibuffer_window);
   defsubr (&Sset_minibuffer_window);
   defsubr (&Sread_from_minibuffer);
diff --git a/src/xdisp.c b/src/xdisp.c
index 49225c56fe..1b1a34b97f 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -11804,7 +11804,7 @@ resize_mini_window (struct window *w, bool exact_p)
 	}
 
       /* Compute a suitable window start.  */
-      if (height > max_height)
+      if (height > max_height && !EQ (Vstart_display_at_beginning_of_minibuffer, Qt))
 	{
 	  height = (max_height / unit) * unit;
 	  init_iterator (&it, w, ZV, ZV_BYTE, NULL, DEFAULT_FACE_ID);
@@ -11812,7 +11812,10 @@ resize_mini_window (struct window *w, bool exact_p)
 	  start = it.current.pos;
 	}
       else
-	SET_TEXT_POS (start, BEGV, BEGV_BYTE);
+	{
+	  if (height > max_height) height = (max_height / unit) * unit;
+	  SET_TEXT_POS (start, BEGV, BEGV_BYTE);
+	}
 
       SET_MARKER_FROM_TEXT_POS (w->start, start);