feat(ui): add scroll_delta to win_viewport event #19270

scroll_delta contains how much the top line of a window moved since the
last time win_viewport was emitted. It is expected to be used to
implement smooth scrolling. For this purpose it only counts "virtual" or
"displayed" so folds should count as one line. Because of this it
adds extra information that cannot be computed from the topline
parameter.

Fixes #19227

10 files changed, 192 insertions, 114 deletions

diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index 78cd05b7d5..48d954fe2d 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -223,6 +223,9 @@ The following changes to existing APIs or features add new behavior.
 
 • API calls now show more information about where an exception happened.
 
+• The `win_viewport` UI event now contains information about virtual lines,
+  meaning that smooth scrolling can now be implemented more consistenlty.
+
 ==============================================================================
 REMOVED FEATURES                                                 *news-removed*
 
diff --git a/runtime/doc/ui.txt b/runtime/doc/ui.txt
index e706e36374..4ff74714f3 100644
--- a/runtime/doc/ui.txt
+++ b/runtime/doc/ui.txt
@@ -626,11 +626,15 @@ tabs.
 	When |ui-messages| is active, no message grid is used, and this event
 	will not be sent.
 
-["win_viewport", grid, win, topline, botline, curline, curcol] ~
+["win_viewport", grid, win, topline, botline, curline, curcol, line_count, scroll_delta] ~
 	Indicates the range of buffer text displayed in the window, as well
 	as the cursor position in the buffer. All positions are zero-based.
 	`botline` is set to one more than the line count of the buffer, if
-	there are filler lines past the end.
+	there are filler lines past the end. `scroll_delta` contains how much
+	the top line of a window moved since `win_viewport` was last emitted.
+	It is intended to be used to implement smooth scrolling. For this
+	purpose it only counts "virtual" or "displayed" lines, so folds
+	only count as one line.
 
 ["win_extmark", grid, win, ns_id, mark_id, row, col] ~
 	Updates the position of an extmark which is currently visible in a
diff --git a/src/nvim/api/ui_events.in.h b/src/nvim/api/ui_events.in.h
index a08e8dbfeb..b93f521ca3 100644
--- a/src/nvim/api/ui_events.in.h
+++ b/src/nvim/api/ui_events.in.h
@@ -114,7 +114,7 @@ void msg_set_pos(Integer grid, Integer row, Boolean scrolled, String sep_char)
   FUNC_API_SINCE(6) FUNC_API_COMPOSITOR_IMPL FUNC_API_CLIENT_IGNORE;
 
 void win_viewport(Integer grid, Window win, Integer topline, Integer botline, Integer curline,
-                  Integer curcol, Integer line_count)
+                  Integer curcol, Integer line_count, Integer scroll_delta)
   FUNC_API_SINCE(7) FUNC_API_CLIENT_IGNORE;
 
 void win_extmark(Integer grid, Window win, Integer ns_id, Integer mark_id, Integer row, Integer col)
diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index 6122136a75..4bdf5aac64 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -1222,6 +1222,7 @@ struct window_S {
   colnr_T w_valid_leftcol;          // last known w_leftcol
 
   bool w_viewport_invalid;
+  linenr_T w_viewport_last_topline;  // topline when the viewport was last updated
 
   // w_cline_height is the number of physical lines taken by the buffer line
   // that the cursor is on.  We use this to avoid extra calls to plines_win().
diff --git a/src/nvim/window.c b/src/nvim/window.c
index 38b7326363..39e9efbabb 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -993,10 +993,22 @@ void ui_ext_win_viewport(win_T *wp)
       // interact with incomplete final line? Diff filler lines?
       botline = wp->w_buffer->b_ml.ml_line_count;
     }
+    int scroll_delta = 0;
+    if (wp->w_viewport_last_topline > line_count) {
+      scroll_delta -= wp->w_viewport_last_topline - line_count;
+      wp->w_viewport_last_topline = line_count;
+    }
+    if (wp->w_topline < wp->w_viewport_last_topline) {
+      scroll_delta -= plines_m_win(wp, wp->w_topline, wp->w_viewport_last_topline - 1);
+    } else if (wp->w_topline > wp->w_viewport_last_topline
+               && wp->w_topline <= line_count) {
+      scroll_delta += plines_m_win(wp, wp->w_viewport_last_topline, wp->w_topline - 1);
+    }
     ui_call_win_viewport(wp->w_grid_alloc.handle, wp->handle, wp->w_topline - 1,
                          botline, wp->w_cursor.lnum - 1, wp->w_cursor.col,
-                         line_count);
+                         line_count, scroll_delta);
     wp->w_viewport_invalid = false;
+    wp->w_viewport_last_topline = wp->w_topline;
   }
 }
 
@@ -5038,6 +5050,7 @@ static win_T *win_alloc(win_T *after, bool hidden)
   new_wp->w_floating = 0;
   new_wp->w_float_config = FLOAT_CONFIG_INIT;
   new_wp->w_viewport_invalid = true;
+  new_wp->w_viewport_last_topline = 1;
 
   new_wp->w_ns_hl = -1;
 
diff --git a/test/functional/ui/cmdline_spec.lua b/test/functional/ui/cmdline_spec.lua
index 1c9ac7f7ba..a9469bdf2d 100644
--- a/test/functional/ui/cmdline_spec.lua
+++ b/test/functional/ui/cmdline_spec.lua
@@ -1241,7 +1241,7 @@ describe('cmdheight=0', function()
       {1:~                        }|
     ## grid 3
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+      [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
     }}
 
     feed '/p'
@@ -1261,7 +1261,7 @@ describe('cmdheight=0', function()
     ## grid 3
       /p^                       |
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+      [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
     }}
   end)
 
diff --git a/test/functional/ui/float_spec.lua b/test/functional/ui/float_spec.lua
index 4e691512c1..62e8221e87 100644
--- a/test/functional/ui/float_spec.lua
+++ b/test/functional/ui/float_spec.lua
@@ -1433,8 +1433,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = { { id = 1002 }, "NW", 1, 2, 5, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -1476,8 +1476,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = { { id = 1002 }, "NW", 1, 2, 5, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -1519,8 +1519,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = { { id = 1002 }, "NW", 1, 2, 5, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -1562,8 +1562,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = { { id = 1002 }, "NW", 1, 2, 5, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -1606,8 +1606,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = { { id = 1002 }, "NW", 1, 2, 5, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -1647,8 +1647,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = { { id = 1002 }, "NW", 1, 2, 5, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -1688,8 +1688,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = { { id = 1002 }, "NW", 1, 2, 5, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -1731,8 +1731,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = { { id = 1002 }, "NW", 1, 2, 5, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -1781,8 +1781,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = { { id = 1002 }, "NW", 1, 2, 5, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 6, curline = 5, curcol = 0, linecount = 6};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2};
+          [2] = {win = {id = 1000}, topline = 0, botline = 6, curline = 5, curcol = 0, linecount = 6, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -1868,8 +1868,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = { { id = 1002 }, "NW", 1, 2, 5, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -1911,8 +1911,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = { { id = 1002 }, "NW", 1, 2, 5, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -1954,8 +1954,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = { { id = 1002 }, "NW", 1, 2, 5, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -1997,8 +1997,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = { { id = 1002 }, "NW", 1, 2, 5, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -2048,8 +2048,8 @@ describe('float window', function()
         ]], float_pos={
           [4] = { { id = 1001 }, "NW", 1, 0, 0, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -2106,8 +2106,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = { { id = 1002 }, "NW", 1, 0, 5, true }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 2, curcol = 0, linecount = 3};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 2, curcol = 0, linecount = 3, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -2165,8 +2165,8 @@ describe('float window', function()
           [5] = { { id = 1002 }, "NW", 1, 0, 5, true, 50 },
           [6] = { { id = -1 }, "NW", 5, 4, 0, false, 100 }
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount=1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 2, curcol = 3, linecount=3};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount=1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 2, curcol = 3, linecount=3, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -2618,7 +2618,8 @@ describe('float window', function()
               curline = 0,
               curcol = 3,
               linecount = 2,
-              win = { id = 1000 }
+              sum_scroll_delta = 0,
+              win = { id = 1000 },
           },
           [4] = {
               topline = 0,
@@ -2626,6 +2627,7 @@ describe('float window', function()
               curline = 0,
               curcol = 3,
               linecount = 2,
+              sum_scroll_delta = 0,
               win = { id = 1001 }
           },
           [5] = {
@@ -2634,6 +2636,7 @@ describe('float window', function()
             curline = 0,
             curcol = 0,
             linecount = 1,
+            sum_scroll_delta = 0,
             win = { id = 1002 }
           }
         }}
@@ -7149,8 +7152,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = {{id = 1002}, "NW", 1, 2, 5, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 0, curcol = 0, linecount = 3};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 0, curcol = 0, linecount = 3, sum_scroll_delta = 0};
         }}
 
         meths.input_mouse('left', 'press', '', 5, 0, 0)
@@ -7179,8 +7182,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = {{id = 1002}, "NW", 1, 2, 5, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 0, curcol = 0, linecount = 3};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 0, curcol = 0, linecount = 3, sum_scroll_delta = 0};
         }}
 
         meths.input_mouse('left', 'drag', '', 5, 1, 2)
@@ -7209,8 +7212,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = {{id = 1002}, "NW", 1, 2, 5, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 1, curcol = 2, linecount = 3};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 1, curcol = 2, linecount = 3, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -7279,8 +7282,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = {{id = 1002}, "NW", 1, 0, 5, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 0, curcol = 0, linecount = 3};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 0, curcol = 0, linecount = 3, sum_scroll_delta = 0};
         }}
 
         meths.input_mouse('left', 'press', '', 5, 1, 1)
@@ -7311,8 +7314,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = {{id = 1002}, "NW", 1, 0, 5, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 0, curcol = 0, linecount = 3};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 0, curcol = 0, linecount = 3, sum_scroll_delta = 0};
         }}
 
         meths.input_mouse('left', 'drag', '', 5, 2, 3)
@@ -7343,8 +7346,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = {{id = 1002}, "NW", 1, 0, 5, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 1, curcol = 2, linecount = 3};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 1, curcol = 2, linecount = 3, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -7413,8 +7416,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = {{id = 1002}, "NW", 1, 1, 5, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 0, curcol = 0, linecount = 3};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 0, curcol = 0, linecount = 3, sum_scroll_delta = 0};
         }}
 
         meths.input_mouse('left', 'press', '', 5, 1, 0)
@@ -7444,8 +7447,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = {{id = 1002}, "NW", 1, 1, 5, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 0, curcol = 0, linecount = 3};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 0, curcol = 0, linecount = 3, sum_scroll_delta = 0};
         }}
 
         meths.input_mouse('left', 'drag', '', 5, 2, 2)
@@ -7475,8 +7478,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = {{id = 1002}, "NW", 1, 1, 5, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 1, curcol = 2, linecount = 3};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 3, curline = 1, curcol = 2, linecount = 3, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -8228,8 +8231,8 @@ describe('float window', function()
         ]], float_pos={
           [4] = { { id = 1001 }, "NW", 1, 2, 5, true };
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -8285,10 +8288,10 @@ describe('float window', function()
           [5] = { { id = 1002 }, "NW", 1, 3, 8, true };
           [6] = { { id = 1003 }, "NW", 1, 4, 10, true };
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount=1};
-          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount=1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount=1};
-          [6] = {win = {id = 1003}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount=1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount=1, sum_scroll_delta = 0};
+          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount=1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount=1, sum_scroll_delta = 0};
+          [6] = {win = {id = 1003}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount=1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -8333,8 +8336,8 @@ describe('float window', function()
         ]], float_pos={
           [4] = { { id = 1001 }, "NW", 1, 2, 5, true };
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -8390,10 +8393,10 @@ describe('float window', function()
           [5] = { { id = 1002 }, "NW", 1, 4, 10, true };
           [6] = { { id = 1003 }, "NW", 1, 3, 8, true };
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [6] = {win = {id = 1003}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [6] = {win = {id = 1003}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -8453,10 +8456,10 @@ describe('float window', function()
           [5] = {{id = 1002}, "NW", 1, 2, 6, true, 50};
           [6] = {{id = 1003}, "NW", 1, 3, 7, true, 40};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [6] = {win = {id = 1003}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [6] = {win = {id = 1003}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -8502,8 +8505,8 @@ describe('float window', function()
         ]], float_pos={
           [4] = {{id = 1001}, "NW", 1, 1, 5, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -8549,8 +8552,8 @@ describe('float window', function()
         ]], float_pos={
           [4] = {{id = 1001}, "NW", 1, 0, 4, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -8604,8 +8607,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = {{id = 1002}, "SW", 1, 9, 0, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -8658,8 +8661,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = {{id = 1002}, "SW", 1, 9, 0, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -8701,7 +8704,7 @@ describe('float window', function()
           {0:~                                       }|
         ## grid 3
         ]], win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -8756,8 +8759,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = {{id = 1002}, "SW", 1, 8, 0, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -8816,8 +8819,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = {{id = 1002}, "SW", 1, 8, 0, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -8865,8 +8868,8 @@ describe('float window', function()
         ]], float_pos={
           [5] = {{id = 1002}, "SW", 1, 8, 0, true, 50};
         }, win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
@@ -8907,7 +8910,7 @@ describe('float window', function()
         ## grid 3
                                                   |
         ]], win_viewport={
-          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
         }}
       else
         screen:expect{grid=[[
diff --git a/test/functional/ui/multigrid_spec.lua b/test/functional/ui/multigrid_spec.lua
index 71adeb42a4..2525314b8e 100644
--- a/test/functional/ui/multigrid_spec.lua
+++ b/test/functional/ui/multigrid_spec.lua
@@ -2127,7 +2127,7 @@ describe('ext_multigrid', function()
     ## grid 3
                                                       |
     ]], win_viewport={
-      [2] = {win = { id = 1000 }, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1}
+      [2] = {win = { id = 1000 }, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0}
     }}
     insert([[
       Lorem ipsum dolor sit amet, consectetur
@@ -2162,7 +2162,7 @@ describe('ext_multigrid', function()
     ## grid 3
                                                       |
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 5, botline = 11, curline = 10, curcol = 7, linecount = 11},
+      [2] = {win = {id = 1000}, topline = 5, botline = 11, curline = 10, curcol = 7, linecount = 11, sum_scroll_delta = 5},
     }}
 
 
@@ -2187,7 +2187,7 @@ describe('ext_multigrid', function()
     ## grid 3
                                                       |
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 2, botline = 9, curline = 7, curcol = 0, linecount = 11},
+      [2] = {win = {id = 1000}, topline = 2, botline = 9, curline = 7, curcol = 0, linecount = 11, sum_scroll_delta = 2},
     }}
 
     command("split")
@@ -2211,8 +2211,8 @@ describe('ext_multigrid', function()
       reprehenderit in voluptate velit esse cillum    |
       ^dolore eu fugiat nulla pariatur. Excepteur sint |
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 6, botline = 9, curline = 7, curcol = 0, linecount = 11},
-      [4] = {win = {id = 1001}, topline = 5, botline = 9, curline = 7, curcol = 0, linecount = 11},
+      [2] = {win = {id = 1000}, topline = 6, botline = 9, curline = 7, curcol = 0, linecount = 11, sum_scroll_delta = 6},
+      [4] = {win = {id = 1001}, topline = 5, botline = 9, curline = 7, curcol = 0, linecount = 11, sum_scroll_delta = 5},
     }}
 
     feed("b")
@@ -2236,8 +2236,8 @@ describe('ext_multigrid', function()
       reprehenderit in voluptate velit esse ^cillum    |
       dolore eu fugiat nulla pariatur. Excepteur sint |
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 6, botline = 9, curline = 7, curcol = 0, linecount = 11},
-      [4] = {win = {id = 1001}, topline = 5, botline = 9, curline = 6, curcol = 38, linecount = 11},
+      [2] = {win = {id = 1000}, topline = 6, botline = 9, curline = 7, curcol = 0, linecount = 11, sum_scroll_delta = 6},
+      [4] = {win = {id = 1001}, topline = 5, botline = 9, curline = 6, curcol = 38, linecount = 11, sum_scroll_delta = 5},
     }}
 
     feed("2k")
@@ -2261,8 +2261,8 @@ describe('ext_multigrid', function()
       ea commodo consequat. Duis aute irure dolor in  |
       reprehenderit in voluptate velit esse cillum    |
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 6, botline = 9, curline = 7, curcol = 0, linecount = 11},
-      [4] = {win = {id = 1001}, topline = 4, botline = 8, curline = 4, curcol = 38, linecount = 11},
+      [2] = {win = {id = 1000}, topline = 6, botline = 9, curline = 7, curcol = 0, linecount = 11, sum_scroll_delta = 6},
+      [4] = {win = {id = 1001}, topline = 4, botline = 8, curline = 4, curcol = 38, linecount = 11, sum_scroll_delta = 4},
     }}
 
     -- handles non-current window
@@ -2287,8 +2287,59 @@ describe('ext_multigrid', function()
       ea commodo consequat. Duis aute irure dolor in  |
       reprehenderit in voluptate velit esse cillum    |
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 0, botline = 3, curline = 0, curcol = 10, linecount = 11},
-      [4] = {win = {id = 1001}, topline = 4, botline = 8, curline = 4, curcol = 38, linecount = 11},
+      [2] = {win = {id = 1000}, topline = 0, botline = 3, curline = 0, curcol = 10, linecount = 11, sum_scroll_delta = 0},
+      [4] = {win = {id = 1001}, topline = 4, botline = 8, curline = 4, curcol = 38, linecount = 11, sum_scroll_delta = 4},
+    }}
+
+    -- sum_scroll_delta works with folds
+    feed('zfj')
+    screen:expect{grid=[[
+    ## grid 1
+      [4:------------------------------------------------]|
+      [4:------------------------------------------------]|
+      [4:------------------------------------------------]|
+      {11:[No Name] [+]                                   }|
+      [2:------------------------------------------------]|
+      [2:------------------------------------------------]|
+      {12:[No Name] [+]                                   }|
+      [3:------------------------------------------------]|
+    ## grid 2
+      Lorem ipsum dolor sit amet, consectetur         |
+      adipisicing elit, sed do eiusmod tempor         |
+    ## grid 3
+                                                      |
+    ## grid 4
+      {13:^+--  2 lines: exercitation ullamco laboris nisi }|
+      reprehenderit in voluptate velit esse cillum    |
+      dolore eu fugiat nulla pariatur. Excepteur sint |
+    ]], win_viewport={
+      [2] = {win = {id = 1000}, topline = 0, botline = 3, curline = 0, curcol = 10, linecount = 11, sum_scroll_delta = 0},
+      [4] = {win = {id = 1001}, topline = 4, botline = 9, curline = 4, curcol = 38, linecount = 11, sum_scroll_delta = 4},
+    }}
+
+    feed('<c-e>')
+    screen:expect{grid=[[
+    ## grid 1
+      [4:------------------------------------------------]|
+      [4:------------------------------------------------]|
+      [4:------------------------------------------------]|
+      {11:[No Name] [+]                                   }|
+      [2:------------------------------------------------]|
+      [2:------------------------------------------------]|
+      {12:[No Name] [+]                                   }|
+      [3:------------------------------------------------]|
+    ## grid 2
+      Lorem ipsum dolor sit amet, consectetur         |
+      adipisicing elit, sed do eiusmod tempor         |
+    ## grid 3
+                                                      |
+    ## grid 4
+      ^reprehenderit in voluptate velit esse cillum    |
+      dolore eu fugiat nulla pariatur. Excepteur sint |
+      occaecat cupidatat non proident, sunt in culpa  |
+    ]], win_viewport={
+      [2] = {win = {id = 1000}, topline = 0, botline = 3, curline = 0, curcol = 10, linecount = 11, sum_scroll_delta = 0},
+      [4] = {win = {id = 1001}, topline = 6, botline = 10, curline = 6, curcol = 0, linecount = 11, sum_scroll_delta = 5},
     }}
   end)
 
@@ -2314,7 +2365,7 @@ describe('ext_multigrid', function()
     ## grid 3
                                                       |
     ]], win_viewport={
-      [2] = {win = { id = 1000 }, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1}
+      [2] = {win = { id = 1000 }, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0}
     }}
     insert([[
       Lorem ipsum dolor sit amet, consectetur
@@ -2349,7 +2400,7 @@ describe('ext_multigrid', function()
     ## grid 3
                                                       |
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 5, botline = 11, curline = 10, curcol = 7, linecount = 11},
+      [2] = {win = {id = 1000}, topline = 5, botline = 11, curline = 10, curcol = 7, linecount = 11, sum_scroll_delta = 5},
     }}
 
     meths.input_mouse('left', 'press', '', 1,5, 1)
@@ -2376,7 +2427,7 @@ describe('ext_multigrid', function()
     ## grid 3
       {7:-- VISUAL --}                                    |
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 6, botline = 12, curline = 10, curcol = 1, linecount = 11},
+      [2] = {win = {id = 1000}, topline = 6, botline = 12, curline = 10, curcol = 1, linecount = 11, sum_scroll_delta = 6},
     }}
   end)
 
@@ -2414,8 +2465,8 @@ describe('ext_multigrid', function()
       {1:~                                                    }|
       {1:~                                                    }|
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-      [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+      [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+      [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
     }}
 
     -- XXX: hack to get notifications. Could use next_msg() also.
@@ -2459,8 +2510,8 @@ describe('ext_multigrid', function()
       {1:~                                                    }|
       {1:~                                                    }|
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-      [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+      [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+      [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
     }}
     eq({}, win_pos)
 
@@ -2497,8 +2548,8 @@ describe('ext_multigrid', function()
       {1:~                                                    }|
       {1:~                                                    }|
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
-      [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+      [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+      [4] = {win = {id = 1001}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
     }}
     eq({}, win_pos)
   end)
diff --git a/test/functional/ui/screen.lua b/test/functional/ui/screen.lua
index e5a449fa66..f5ae9c8e89 100644
--- a/test/functional/ui/screen.lua
+++ b/test/functional/ui/screen.lua
@@ -803,14 +803,17 @@ function Screen:_handle_win_pos(grid, win, startrow, startcol, width, height)
   self.float_pos[grid] = nil
 end
 
-function Screen:_handle_win_viewport(grid, win, topline, botline, curline, curcol, linecount)
+function Screen:_handle_win_viewport(grid, win, topline, botline, curline, curcol, linecount, scroll_delta)
+  -- accumulate scroll delta
+  local last_scroll_delta = self.win_viewport[grid] and self.win_viewport[grid].sum_scroll_delta or 0
   self.win_viewport[grid] = {
     win = win,
     topline = topline,
     botline = botline,
     curline = curline,
     curcol = curcol,
-    linecount = linecount
+    linecount = linecount,
+    sum_scroll_delta = scroll_delta + last_scroll_delta
   }
 end
 
@@ -1348,7 +1351,7 @@ local function fmt_ext_state(name, state)
     for k,v in pairs(state) do
       str = (str.."  ["..k.."] = {win = {id = "..v.win.id.."}, topline = "
              ..v.topline..", botline = "..v.botline..", curline = "..v.curline
-             ..", curcol = "..v.curcol..", linecount = "..v.linecount.."};\n")
+             ..", curcol = "..v.curcol..", linecount = "..v.linecount..", scroll_delta = "..v.scroll_delta.."};\n")
     end
     return str .. "}"
   elseif name == "float_pos" then
diff --git a/test/functional/ui/searchhl_spec.lua b/test/functional/ui/searchhl_spec.lua
index 916a5eb537..3c8dceb8cb 100644
--- a/test/functional/ui/searchhl_spec.lua
+++ b/test/functional/ui/searchhl_spec.lua
@@ -52,7 +52,7 @@ describe('search highlighting', function()
       {1:~                                       }|
       /text^                                   |
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 0, botline = 3, curline = 0, curcol = 9, linecount = 2};
+      [2] = {win = {id = 1000}, topline = 0, botline = 3, curline = 0, curcol = 9, linecount = 2, sum_scroll_delta = 0};
     }}
   end)
 
@@ -616,7 +616,7 @@ describe('search highlighting', function()
       {1:~                                       }|
       {4:search hit BOTTOM, continuing at TOP}    |
     ]], win_viewport={
-      [2] = {win = {id = 1000}, topline = 0, botline = 3, curline = 0, curcol = 11, linecount = 2};
+      [2] = {win = {id = 1000}, topline = 0, botline = 3, curline = 0, curcol = 11, linecount = 2, sum_scroll_delta = 0};
     }}
 
     -- check highlights work also in folds