feat: implement a working WinScrolled autocmd

src/nvim/auevents.lua

@@ -72,7 +72,6 @@ return {
     'QuickFixCmdPre',         -- before :make, :grep etc.
     'QuitPre',                -- before :quit
     'RemoteReply',            -- upon string reception from a remote vim
-    'Scroll',                 -- after scrolling
     'SessionLoadPost',        -- after loading a session file
     'ShellCmdPost',           -- after ":!cmd"
     'ShellFilterPost',        -- after ":1,2!cmd", ":w !cmd", ":r !cmd".
@@ -113,6 +112,7 @@ return {
     'WinEnter',               -- after entering a window
     'WinLeave',               -- before leaving a window
     'WinNew',                 -- when entering a new window
+    'WinScrolled',            -- after scrolling a window
   },
   aliases = {
     BufCreate = 'BufAdd',
@@ -124,7 +124,6 @@ return {
   -- syntax file
   nvim_specific = {
     DirChanged=true,
-    Scroll=true,
     Signal=true,
     TabClosed=true,
     TabNew=true,
@@ -134,5 +133,6 @@ return {
     UIEnter=true,
     UILeave=true,
     WinClosed=true,
+    WinScrolled=true,
   },
 }

src/nvim/buffer_defs.h

@@ -1204,6 +1204,15 @@ struct window_S {
   colnr_T w_skipcol;                // starting column when a single line
                                     // doesn't fit in the window
 
+  /*
+   * "w_last_topline" and "w_last_leftcol" are used to determine if
+   * a Scroll autocommand should be emitted.
+   */
+  linenr_T w_last_topline;          ///< last known value for topline
+  colnr_T w_last_leftcol;          ///< last known value for leftcol
+  int w_last_width;                 ///< last known value for width
+  int w_last_height;                ///< last known value for height
+
   //
   // Layout of the window in the screen.
   // May need to add "msg_scrolled" to "w_winrow" in rare situations.

src/nvim/edit.c

@@ -1482,9 +1482,10 @@ static void ins_redraw(
     }
   }
 
-  if (ready && has_event(EVENT_SCROLL)
-      && curwin->w_viewport_invalid) {
-    apply_autocmds(EVENT_SCROLL, NULL, NULL, false, curbuf);
+  // Trigger Scroll if viewport changed.
+  if (ready && has_event(EVENT_WINSCROLLED)
+      && win_did_scroll(curwin)) {
+    do_autocmd_winscrolled(curwin);
   }
 
   if (curwin->w_p_cole > 0 && conceal_cursor_line(curwin)

src/nvim/normal.c

@@ -1195,10 +1195,10 @@ static void normal_check_interrupt(NormalState *s)
 
 static void normal_check_window_scrolled(NormalState *s)
 {
-  // Trigger Scroll if the window moved.
-  if (!finish_op && has_event(EVENT_SCROLL)
-      && curwin->w_viewport_invalid) {
-    apply_autocmds(EVENT_SCROLL, NULL, NULL, false, curbuf);
+  // Trigger Scroll if the viewport changed.
+  if (!finish_op && has_event(EVENT_WINSCROLLED)
+      && win_did_scroll(curwin)) {
+    do_autocmd_winscrolled(curwin);
   }
 }
 
@@ -1325,6 +1325,10 @@ static int normal_check(VimState *state)
   if (skip_redraw || exmode_active) {
     skip_redraw = false;
   } else if (do_redraw || stuff_empty()) {
+    // Need to make sure w_topline and w_leftcol are correct before
+    // normal_check_window_scrolled() is called.
+    update_topline();
+
     normal_check_cursor_moved(s);
     normal_check_text_changed(s);
     normal_check_window_scrolled(s);

src/nvim/window.c

@@ -4965,6 +4965,30 @@ void shell_new_columns(void)
   win_reconfig_floats();  // The size of floats might change
 }
 
+/*
+ * Check if "wp" has scrolled since last time it was checked
+ */
+bool win_did_scroll(win_T *wp)
+{
+  return (curwin->w_last_topline != curwin->w_topline ||
+          curwin->w_last_leftcol != curwin->w_leftcol ||
+          curwin->w_last_width != curwin->w_width ||
+          curwin->w_last_height != curwin->w_height);
+}
+
+/*
+ * Trigger WinScrolled autocmd
+ */
+void do_autocmd_winscrolled(win_T *wp)
+{
+  apply_autocmds(EVENT_WINSCROLLED, NULL, NULL, false, curbuf);
+
+  wp->w_last_topline = wp->w_topline;
+  wp->w_last_leftcol = wp->w_leftcol;
+  wp->w_last_width = wp->w_width;
+  wp->w_last_height = wp->w_height;
+}
+
 /*
  * Save the size of all windows in "gap".
  */