fix(messages): reset msg_grid_scroll_discount when redrawing (#21000)

src/nvim/drawscreen.c

@@ -455,6 +455,7 @@ int update_screen(void)
     }
     msg_scrolled = 0;
     msg_scrolled_at_flush = 0;
+    msg_grid_scroll_discount = 0;
     need_wait_return = false;
   }
 

src/nvim/message.c

@@ -130,7 +130,6 @@ static bool msg_ext_history_visible = false;
 static bool msg_ext_keep_after_cmdline = false;
 
 static int msg_grid_pos_at_flush = 0;
-static int msg_grid_scroll_discount = 0;
 
 static void ui_ext_msg_set_pos(int row, bool scrolled)
 {
@@ -2445,6 +2444,7 @@ void msg_reset_scroll(void)
   }
   msg_scrolled = 0;
   msg_scrolled_at_flush = 0;
+  msg_grid_scroll_discount = 0;
 }
 
 /// Increment "msg_scrolled".

src/nvim/message.h

@@ -66,6 +66,8 @@ EXTERN ScreenGrid msg_grid_adj INIT(= SCREEN_GRID_INIT);
 // value of msg_scrolled at latest msg_scroll_flush.
 EXTERN int msg_scrolled_at_flush INIT(= 0);
 
+EXTERN int msg_grid_scroll_discount INIT(= 0);
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "message.h.generated.h"
 #endif