Avoid leaving artifacts when the system caret is used on w32

* src/xdisp.c (try_window_reusing_current_matrix, try_window_id):
* src/dispnew.c (scrolling_window) [HAVE_NTGUI]: If
w32-use-visible-system-caret is non-nil, disallow scrolling the
display are in scroll_run_hook.  This avoids copying traces of the
caret, about which Emacs knows nothing, and thus considers those
pixels show the default background.  (Bug#39188)
(gui_update_window_end): Block input only around part of the code,
as we did before this code was extracted from backend-specific
implementations.

* src/w32term.c (w32_update_window_begin, w32_update_window_end):
Only hide/show the caret when redisplaying the window where the
caret is shown.

Author: Eli-Zaretskii

src/dispnew.c

@@ -3743,11 +3743,10 @@ gui_update_window_end (struct window *w, bool cursor_on_p,
 {
   struct frame *f = XFRAME (WINDOW_FRAME (w));
 
-  block_input ();
-
   /* Pseudo windows don't have cursors, so don't display them here.  */
   if (!w->pseudo_window_p)
     {
+      block_input ();
 
       if (cursor_on_p)
 	display_and_set_cursor (w, true,
@@ -3761,6 +3760,7 @@ gui_update_window_end (struct window *w, bool cursor_on_p,
 	  else
 	    gui_draw_vertical_border (w);
 	}
+      unblock_input ();
     }
 
   /* If a row with mouse-face was overwritten, arrange for
@@ -3778,7 +3778,6 @@ gui_update_window_end (struct window *w, bool cursor_on_p,
     FRAME_RIF (f)->update_window_end_hook (w,
                                            cursor_on_p,
                                            mouse_face_overwritten_p);
-  unblock_input ();
 }
 
 #endif /* HAVE_WINDOW_SYSTEM  */
@@ -4360,6 +4359,14 @@ scrolling_window (struct window *w, int tab_line_p)
     return 0;
 #endif
 
+  /* Can't scroll the display of w32 GUI frames when position of point
+     is indicated by the system caret, because scrolling the display
+     will then "copy" the pixles used by the caret.  */
+#ifdef HAVE_NTGUI
+  if (w32_use_visible_system_caret)
+    return 0;
+#endif
+
   /* Give up if some rows in the desired matrix are not enabled.  */
   if (! MATRIX_ROW_ENABLED_P (desired_matrix, i))
     return -1;

src/w32term.c

@@ -560,7 +560,8 @@ static void
 w32_update_window_begin (struct window *w)
 {
   /* Hide the system caret during an update.  */
-  if (w32_use_visible_system_caret && w32_system_caret_hwnd)
+  if (w32_use_visible_system_caret && w32_system_caret_hwnd
+      && w == w32_system_caret_window)
     {
       SendMessageTimeout (w32_system_caret_hwnd, WM_EMACS_HIDE_CARET, 0, 0,
 			  0, 6000, NULL);
@@ -657,7 +658,8 @@ w32_update_window_end (struct window *w, bool cursor_on_p,
   /* Unhide the caret.  This won't actually show the cursor, unless it
      was visible before the corresponding call to HideCaret in
      w32_update_window_begin.  */
-  if (w32_use_visible_system_caret && w32_system_caret_hwnd)
+  if (w32_use_visible_system_caret && w32_system_caret_hwnd
+      && w == w32_system_caret_window)
     {
       SendMessageTimeout (w32_system_caret_hwnd, WM_EMACS_SHOW_CARET, 0, 0,
 			  0, 6000, NULL);

src/xdisp.c

@@ -19191,6 +19191,14 @@ try_window_reusing_current_matrix (struct window *w)
   if (!NILP (Vdisplay_line_numbers))
     return false;
 
+  /* Can't scroll the display of w32 GUI frames when position of point
+     is indicated by the system caret, because scrolling the display
+     will then "copy" the pixles used by the caret.  */
+#ifdef HAVE_NTGUI
+  if (w32_use_visible_system_caret)
+    return false;
+#endif
+
   /* The variable new_start now holds the new window start.  The old
      start `start' can be determined from the current matrix.  */
   SET_TEXT_POS_FROM_MARKER (new_start, w->start);
@@ -20175,6 +20183,15 @@ try_window_id (struct window *w)
   if (MATRIX_ROW_START_CHARPOS (row) == MATRIX_ROW_END_CHARPOS (row))
     GIVE_UP (20);
 
+  /* Can't let scroll_run_hook below run on w32 GUI frames when
+     position of point is indicated by the system caret, because
+     scrolling the display will then "copy" the pixles used by the
+     caret.  */
+#ifdef HAVE_NTGUI
+  if (FRAME_W32_P (f) && w32_use_visible_system_caret)
+    GIVE_UP (25);
+#endif
+
   /* Compute the position at which we have to start displaying new
      lines.  Some of the lines at the top of the window might be
      reusable because they are not displaying changed text.  Find the