6 files changed, 72 insertions, 6 deletions

diff --git a/src/nvim/auevents.lua b/src/nvim/auevents.lua
index 10647c01a4..9c28398f5b 100644
--- a/src/nvim/auevents.lua
+++ b/src/nvim/auevents.lua
@@ -113,6 +113,7 @@ 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',
@@ -133,5 +134,6 @@ return {
     UIEnter=true,
     UILeave=true,
     WinClosed=true,
+    WinScrolled=true,
   },
 }
diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index 4efc341875..540542f409 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -1230,6 +1230,15 @@ 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.
diff --git a/src/nvim/edit.c b/src/nvim/edit.c
index 3e62ed9036..d7cca9ba36 100644
--- a/src/nvim/edit.c
+++ b/src/nvim/edit.c
@@ -1482,6 +1482,12 @@ static void ins_redraw(
     }
   }
 
+  // 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)
       && conceal_cursor_moved) {
     redrawWinline(curwin, curwin->w_cursor.lnum);
diff --git a/src/nvim/move.c b/src/nvim/move.c
index 98a7792a09..218dcd289d 100644
--- a/src/nvim/move.c
+++ b/src/nvim/move.c
@@ -1023,7 +1023,7 @@ void textpos2screenpos(win_T *wp, pos_T *pos, int *rowp, int *scolp,
 /*
  * Scroll the current window down by "line_count" logical lines.  "CTRL-Y"
  */
-void 
+bool 
 scrolldown (
     long line_count,
     int byfold              /* true: count a closed fold as one line */
@@ -1098,17 +1098,21 @@ scrolldown (
     foldAdjustCursor();
     coladvance(curwin->w_curswant);
   }
+  return moved;
 }
 
 /*
  * Scroll the current window up by "line_count" logical lines.  "CTRL-E"
  */
-void 
+bool 
 scrollup (
     long line_count,
     int byfold              /* true: count a closed fold as one line */
 )
 {
+  linenr_T topline = curwin->w_topline;
+  linenr_T botline = curwin->w_botline;
+
   if ((byfold && hasAnyFolding(curwin))
       || curwin->w_p_diff) {
     // count each sequence of folded lines as one logical line
@@ -1151,6 +1155,12 @@ scrollup (
       ~(VALID_WROW|VALID_WCOL|VALID_CHEIGHT|VALID_CROW|VALID_VIRTCOL);
     coladvance(curwin->w_curswant);
   }
+
+  bool moved = 
+    topline != curwin->w_topline ||
+    botline != curwin->w_botline;
+
+  return moved;
 }
 
 /*
diff --git a/src/nvim/normal.c b/src/nvim/normal.c
index 11e8d354e4..b09b99cf35 100644
--- a/src/nvim/normal.c
+++ b/src/nvim/normal.c
@@ -1197,6 +1197,15 @@ static void normal_check_interrupt(NormalState *s)
   }
 }
 
+static void normal_check_window_scrolled(NormalState *s)
+{
+  // Trigger Scroll if the viewport changed.
+  if (!finish_op && has_event(EVENT_WINSCROLLED)
+      && win_did_scroll(curwin)) {
+    do_autocmd_winscrolled(curwin);
+  }
+}
+
 static void normal_check_cursor_moved(NormalState *s)
 {
   // Trigger CursorMoved if the cursor moved.
@@ -1320,8 +1329,13 @@ 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);
 
     // Updating diffs from changed() does not always work properly,
     // esp. updating folds.  Do an update just before redrawing if
@@ -4111,10 +4125,10 @@ void scroll_redraw(int up, long count)
   int prev_topfill = curwin->w_topfill;
   linenr_T prev_lnum = curwin->w_cursor.lnum;
 
-  if (up)
-    scrollup(count, true);
-  else
+  bool moved = up ?
+    scrollup(count, true) :
     scrolldown(count, true);
+
   if (get_scrolloff_value()) {
     // Adjust the cursor position for 'scrolloff'.  Mark w_topline as
     // valid, otherwise the screen jumps back at the end of the file.
@@ -4146,7 +4160,8 @@ void scroll_redraw(int up, long count)
   }
   if (curwin->w_cursor.lnum != prev_lnum)
     coladvance(curwin->w_curswant);
-  curwin->w_viewport_invalid = true;
+  if (moved)
+    curwin->w_viewport_invalid = true;
   redraw_later(curwin, VALID);
 }
 
diff --git a/src/nvim/window.c b/src/nvim/window.c
index 2147bd4fc7..47b6b7e713 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -4976,6 +4976,30 @@ void shell_new_columns(void)
 }
 
 /*
+ * 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".
  */
 void win_size_save(garray_T *gap)