multigrid: add msg_scroll_start and msg_scroll_reset events

src/nvim/api/ui_events.in.h

@@ -83,11 +83,16 @@ void grid_scroll(Integer grid, Integer top, Integer bot,
   FUNC_API_SINCE(5) FUNC_API_REMOTE_IMPL;
 void grid_destroy(Integer grid)
   FUNC_API_SINCE(5) FUNC_API_REMOTE_ONLY;
+
 void win_position(Integer win, Integer grid, Integer startrow,
                   Integer startcol, Integer width, Integer height)
   FUNC_API_SINCE(5) FUNC_API_REMOTE_ONLY;
 void win_hide(Integer win, Integer grid)
   FUNC_API_SINCE(5) FUNC_API_REMOTE_ONLY;
+void win_scroll_over_start(void)
+  FUNC_API_SINCE(5) FUNC_API_REMOTE_ONLY;
+void win_scroll_over_reset(void)
+  FUNC_API_SINCE(5) FUNC_API_REMOTE_ONLY;
 
 void popupmenu_show(Array items, Integer selected,
                     Integer row, Integer col, Integer grid)

src/nvim/message.c

@@ -1884,6 +1884,9 @@ int msg_scrollsize(void)
  */
 static void msg_scroll_up(void)
 {
+  if (msg_scrolled == 0) {
+    ui_call_win_scroll_over_start();
+  }
   if (dy_flags & DY_MSGSEP) {
     if (msg_scrolled == 0) {
       grid_fill(&default_grid, Rows-p_ch-1, Rows-p_ch, 0, (int)Columns,

src/nvim/screen.c

@@ -292,6 +292,7 @@ void update_screen(int type)
    * if the screen was scrolled up when displaying a message, scroll it down
    */
   if (msg_scrolled) {
+    ui_call_win_scroll_over_reset();
     clear_cmdline = true;
     if (dy_flags & DY_MSGSEP) {
       int valid = MAX(Rows - msg_scrollsize(), 0);

test/functional/ui/screen.lua

@@ -646,6 +646,12 @@ function Screen:_handle_grid_resize(grid, width, height)
   }
 end
 
+function Screen:_handle_msg_scroll_start()
+end
+
+function Screen:_handle_msg_scroll_reset()
+end
+
 function Screen:_handle_flush()
 end