Allow precision-scrolling nonselected windows when the minibuffer is resized

* doc/lispref/windows.texi (Vertical Scrolling): Document new
`preserve-vscroll-p' parameter of `set-window-vscroll'.
* etc/NEWS: Announce new parameter.

* lisp/pixel-scroll.el (pixel-scroll-precision-scroll-down-page)
(pixel-scroll-precision-scroll-up-page): Use that parameter when
setting the vscroll.

* src/window.c (window_scroll_pixel_based, Fset_window_vscroll):
Adjust for new parameter.
* src/window.h (struct window): New flag `preserve_vscroll_p'.

* src/xdisp.c (redisplay_window): Preserve the vscroll inside
force_start on frozen windows with that flag set.  (bug#55312)

6 files changed, 43 insertions, 12 deletions

diff --git a/doc/lispref/windows.texi b/doc/lispref/windows.texi
index 97908bea001..57763c146da 100644
--- a/doc/lispref/windows.texi
+++ b/doc/lispref/windows.texi
@@ -5508,7 +5508,7 @@ pixels, rather than in units of the normal line height.
 @end example
 @end defun
 
-@defun set-window-vscroll window lines &optional pixels-p
+@defun set-window-vscroll window lines &optional pixels-p preserve-vscroll-p
 This function sets @var{window}'s vertical scroll position to
 @var{lines}.  If @var{window} is @code{nil}, the selected window is
 used.  The argument @var{lines} should be zero or positive; if not, it
@@ -5530,6 +5530,12 @@ The return value is the result of this rounding.
 
 If @var{pixels-p} is non-@code{nil}, @var{lines} specifies a number of
 pixels.  In this case, the return value is @var{lines}.
+
+Normally, the vscroll does not take effect on windows that aren't the
+@code{minibuffer-scroll-window} or the selected window when the
+mini-window is resized (@pxref{Minibuffer Windows}).  This ``frozen''
+behavior is disabled when the @var{preserve-vscroll-p} parameter is
+non-@code{nil}, which means to set the vscroll as usual.
 @end defun
 
 @defvar auto-window-vscroll
diff --git a/etc/NEWS b/etc/NEWS
index dbdc161a416..5860010f022 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -2100,6 +2100,11 @@ dimensions.
 Specifying a cons as the FROM argument allows to start measuring text
 from a specified amount of pixels above or below a position.
 
++++
+** 'set-window-vscroll' now accepts a new argument PRESERVE-VSCROLL-P.
+This means the vscroll will not be reset when set on a window that is
+"frozen" due to a mini-window being resized.
+
 ** XDG support
 
 ---
diff --git a/lisp/pixel-scroll.el b/lisp/pixel-scroll.el
index b0fe2f56c03..fc7e680c262 100644
--- a/lisp/pixel-scroll.el
+++ b/lisp/pixel-scroll.el
@@ -547,7 +547,7 @@ the height of the current window."
                               (beginning-of-visual-line)
                               (point)))
                       t)
-    (set-window-vscroll nil desired-vscroll t)))
+    (set-window-vscroll nil desired-vscroll t t)))
 
 (defun pixel-scroll-precision-scroll-down (delta)
   "Scroll the current window down by DELTA pixels."
@@ -586,7 +586,7 @@ the height of the current window."
         (goto-char up-point)))
     (let ((current-vscroll (window-vscroll nil t)))
       (setq delta (- delta current-vscroll))
-      (set-window-vscroll nil 0 t)
+      (set-window-vscroll nil 0 t t)
       (when (> delta 0)
         (let* ((start (window-start))
                (dims (window-text-pixel-size nil (cons start (- delta))
@@ -602,7 +602,7 @@ the height of the current window."
             (signal 'beginning-of-buffer nil))
           (setq delta (- delta height))))
       (when (< delta 0)
-        (set-window-vscroll nil (- delta) t)))))
+        (set-window-vscroll nil (- delta) t t)))))
 
 (defun pixel-scroll-precision-interpolate (delta &optional old-window)
   "Interpolate a scroll of DELTA pixels.
diff --git a/src/window.c b/src/window.c
index 72d10f9da23..47c008a643a 100644
--- a/src/window.c
+++ b/src/window.c
@@ -5636,7 +5636,8 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, bool noerror)
 	      if (w->vscroll < 0 && rtop > 0)
 		{
 		  px = max (0, -w->vscroll - min (rtop, -dy));
-		  Fset_window_vscroll (window, make_fixnum (px), Qt);
+		  Fset_window_vscroll (window, make_fixnum (px), Qt,
+				       Qnil);
 		  return;
 		}
 	    }
@@ -5646,7 +5647,8 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, bool noerror)
 	      if (rbot > 0 && (w->vscroll < 0 || vpos == 0))
 		{
 		  px = max (0, -w->vscroll + min (rbot, dy));
-		  Fset_window_vscroll (window, make_fixnum (px), Qt);
+		  Fset_window_vscroll (window, make_fixnum (px), Qt,
+				       Qnil);
 		  return;
 		}
 
@@ -5655,7 +5657,8 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, bool noerror)
 		{
 		  ptrdiff_t spos;
 
-		  Fset_window_vscroll (window, make_fixnum (0), Qt);
+		  Fset_window_vscroll (window, make_fixnum (0), Qt,
+				       Qnil);
 		  /* If there are other text lines above the current row,
 		     move window start to current row.  Else to next row. */
 		  if (rbot > 0)
@@ -5674,7 +5677,7 @@ window_scroll_pixel_based (Lisp_Object window, int n, bool whole, bool noerror)
 	    }
 	}
       /* Cancel previous vscroll.  */
-      Fset_window_vscroll (window, make_fixnum (0), Qt);
+      Fset_window_vscroll (window, make_fixnum (0), Qt, Qnil);
     }
 
   itdata = bidi_shelve_cache ();
@@ -7944,7 +7947,7 @@ optional second arg PIXELS-P means value is measured in pixels.  */)
 
 
 DEFUN ("set-window-vscroll", Fset_window_vscroll, Sset_window_vscroll,
-       2, 3, 0,
+       2, 4, 0,
        doc: /* Set amount by which WINDOW should be scrolled vertically to VSCROLL.
 This takes effect when displaying tall lines or images.
 
@@ -7954,8 +7957,12 @@ optional third arg PIXELS-P non-nil means that VSCROLL is in pixels.
 If PIXELS-P is nil, VSCROLL may have to be rounded so that it
 corresponds to an integral number of pixels.  The return value is the
 result of this rounding.
-If PIXELS-P is non-nil, the return value is VSCROLL.  */)
-  (Lisp_Object window, Lisp_Object vscroll, Lisp_Object pixels_p)
+If PIXELS-P is non-nil, the return value is VSCROLL.
+
+PRESERVE_VSCROLL_P makes setting the start of WINDOW preserve the
+vscroll if its start is "frozen" due to a resized mini-window.  */)
+  (Lisp_Object window, Lisp_Object vscroll, Lisp_Object pixels_p,
+   Lisp_Object preserve_vscroll_p)
 {
   struct window *w = decode_live_window (window);
   struct frame *f = XFRAME (w->frame);
@@ -7984,6 +7991,8 @@ If PIXELS-P is non-nil, the return value is VSCROLL.  */)
 	  /* Mark W for redisplay.  (bug#55299) */
 	  wset_redisplay (w);
 	}
+
+      w->preserve_vscroll_p = !NILP (preserve_vscroll_p);
     }
 
   return Fwindow_vscroll (window, pixels_p);
diff --git a/src/window.h b/src/window.h
index 387a3be36a9..7f7de588463 100644
--- a/src/window.h
+++ b/src/window.h
@@ -445,6 +445,10 @@ struct window
        window.  */
     bool_bf suspend_auto_hscroll : 1;
 
+    /* True if vscroll should be preserved while forcing the start due
+       to a frozen window.  */
+    bool_bf preserve_vscroll_p : 1;
+
     /* Amount by which lines of this window are scrolled in
        y-direction (smooth scrolling).  */
     int vscroll;
diff --git a/src/xdisp.c b/src/xdisp.c
index f09f209b2e3..b9b3c6d1bf3 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -19168,7 +19168,14 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
       int new_vpos = -1;
 
       w->force_start = false;
-      w->vscroll = 0;
+
+      /* The vscroll should be preserved in this case, since
+	 `pixel-scroll-precision-mode' must continue working normally
+	 when a mini-window is resized.  (bug#55312) */
+      if (!w->preserve_vscroll_p || !window_frozen_p (w))
+	w->vscroll = 0;
+
+      w->preserve_vscroll_p = false;
       w->window_end_valid = false;
 
       /* Forget any recorded base line for line number display.  */