diff --git a/runtime/doc/ui.txt b/runtime/doc/ui.txt
index b8d47923ca..5f217c5537 100644
--- a/runtime/doc/ui.txt
+++ b/runtime/doc/ui.txt
@@ -656,6 +656,11 @@ tabs.
 	the screen and copy it to the viewport destination once `win_viewport`
 	is received.
 
+["win_viewport_margins", grid, win, top, bottom, left, right] ~
+	Indicates the margins of a window grid which are _not_ part of the
+	viewport as indicated by the `win_viewport` event. This happens
+	e.g. in the presence of 'winbar' and floating window borders.
+
 ["win_extmark", grid, win, ns_id, mark_id, row, col] ~
 	Updates the position of an extmark which is currently visible in a
 	window. Only emitted if the mark has the `ui_watched` attribute.
diff --git a/src/nvim/api/ui_events.in.h b/src/nvim/api/ui_events.in.h
index c2f02c34f8..2bd8792d71 100644
--- a/src/nvim/api/ui_events.in.h
+++ b/src/nvim/api/ui_events.in.h
@@ -118,6 +118,10 @@ void win_viewport(Integer grid, Window win, Integer topline, Integer botline, In
                   Integer curcol, Integer line_count, Integer scroll_delta)
   FUNC_API_SINCE(7) FUNC_API_CLIENT_IGNORE;
 
+void win_viewport_margins(Integer grid, Window win, Integer top, Integer bottom, Integer left,
+                          Integer right)
+  FUNC_API_SINCE(12) FUNC_API_CLIENT_IGNORE;
+
 void win_extmark(Integer grid, Window win, Integer ns_id, Integer mark_id, Integer row, Integer col)
   FUNC_API_SINCE(10) FUNC_API_REMOTE_ONLY;
 
diff --git a/src/nvim/window.c b/src/nvim/window.c
index c9c2124730..6d85e088b5 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -6744,6 +6744,13 @@ void win_set_inner_size(win_T *wp, bool valid_cursor)
   wp->w_width_outer = (wp->w_width_inner + win_border_width(wp));
   wp->w_winrow_off = wp->w_border_adj[0] + wp->w_winbar_height;
   wp->w_wincol_off = wp->w_border_adj[3];
+
+  if (ui_has(kUIMultigrid)) {
+    ui_call_win_viewport_margins(wp->w_grid_alloc.handle, wp->handle,
+                                 wp->w_winrow_off, wp->w_border_adj[2],
+                                 wp->w_wincol_off, wp->w_border_adj[1]);
+  }
+
   wp->w_redr_status = true;
 }
 
diff --git a/test/functional/ui/float_spec.lua b/test/functional/ui/float_spec.lua
index 361ea3e778..e83f3ff57e 100644
--- a/test/functional/ui/float_spec.lua
+++ b/test/functional/ui/float_spec.lua
@@ -1536,7 +1536,12 @@ describe('float window', function()
         }, win_viewport={
           [2] = {win = 1000, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
           [4] = {win = 1001, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
-        }}
+        },
+        win_viewport_margins={
+          [2] = {win = 1000, top = 0, bottom = 0, left = 0, right = 0};
+          [4] = {win = 1001, top = 1, bottom = 1, left = 1, right = 1};
+        }
+      }
       else
         screen:expect{grid=[[
           ^                                        |
@@ -1736,7 +1741,12 @@ describe('float window', function()
         }, win_viewport={
           [2] = {win = 1000, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
           [4] = {win = 1001, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
-        }}
+        },
+        win_viewport_margins={
+          [2] = {win = 1000, top = 0, bottom = 0, left = 0, right = 0};
+          [4] = {win = 1001, top = 0, bottom = 0, left = 1, right = 1};
+        }
+      }
       else
         screen:expect{grid=[[
           ^                                        |
@@ -1769,6 +1779,10 @@ describe('float window', function()
         }, win_viewport={
           [2] = {win = 1000, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
           [4] = {win = 1001, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
+        },
+        win_viewport_margins={
+          [2] = {win = 1000, top = 0, bottom = 0, left = 0, right = 0};
+          [4] = {win = 1001, top = 1, bottom = 1, left = 0, right = 0};
         }}
       else
         screen:expect{grid=[[
@@ -1814,6 +1828,10 @@ describe('float window', function()
         }, win_viewport={
           [2] = {win = 1000, topline = 0, botline = 6, curline = 5, curcol = 0, linecount = 6, sum_scroll_delta = 0};
           [4] = {win = 1001, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
+        },
+        win_viewport_margins={
+          [2] = {win = 1000, top = 0, bottom = 0, left = 0, right = 0};
+          [4] = {win = 1001, top = 0, bottom = 1, left = 0, right = 1};
         }}
       else
         screen:expect{grid=[[
@@ -8361,6 +8379,10 @@ describe('float window', function()
         }, win_viewport={
           [2] = {win = 1000, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
           [4] = {win = 1001, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+        },
+        win_viewport_margins={
+          [2] = {win = 1000, top = 0, bottom = 0, left = 0, right = 0};
+          [4] = {win = 1001, top = 2, bottom = 1, left = 1, right = 1};
         }}
       else
         screen:expect{grid=[[
diff --git a/test/functional/ui/multigrid_spec.lua b/test/functional/ui/multigrid_spec.lua
index 4b11a881ed..bb490546f6 100644
--- a/test/functional/ui/multigrid_spec.lua
+++ b/test/functional/ui/multigrid_spec.lua
@@ -2354,6 +2354,9 @@ describe('ext_multigrid', function()
     ]], win_viewport={
       [2] = {win = 1000, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
       [4] = {win = 1001, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+    }, win_viewport_margins={
+      [2] = {win = 1000, top = 0, bottom = 0, left = 0, right = 0};
+      [4] = {win = 1001, top = 0, bottom = 0, left = 0, right = 0};
     }}
 
     -- XXX: hack to get notifications. Could use next_msg() also.
@@ -2384,6 +2387,9 @@ describe('ext_multigrid', function()
     ]], win_viewport={
       [2] = {win = 1000, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
       [4] = {win = 1001, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+    }, win_viewport_margins={
+      [2] = {win = 1000, top = 0, bottom = 0, left = 0, right = 0};
+      [4] = {win = 1001, top = 1, bottom = 0, left = 0, right = 0};
     }}
     eq({}, win_pos)
 
@@ -2406,6 +2412,9 @@ describe('ext_multigrid', function()
     ]], win_viewport={
       [2] = {win = 1000, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
       [4] = {win = 1001, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+    }, win_viewport_margins={
+      [2] = {win = 1000, top = 0, bottom = 0, left = 0, right = 0};
+      [4] = {win = 1001, top = 0, bottom = 0, left = 0, right = 0};
     }}
     eq({}, win_pos)
   end)
diff --git a/test/functional/ui/screen.lua b/test/functional/ui/screen.lua
index d741b440c5..ee9334a9fe 100644
--- a/test/functional/ui/screen.lua
+++ b/test/functional/ui/screen.lua
@@ -204,6 +204,7 @@ function Screen.new(width, height)
     wildmenu_selected = nil,
     win_position = {},
     win_viewport = {},
+    win_viewport_margins = {},
     float_pos = {},
     msg_grid = nil,
     msg_grid_pos = nil,
@@ -337,6 +338,7 @@ local ext_keys = {
   'ruler',
   'float_pos',
   'win_viewport',
+  'win_viewport_margins',
 }
 
 local expect_keys = {
@@ -621,6 +623,9 @@ screen:redraw_debug() to show all intermediate screen states.]]
     if expected.win_viewport == nil then
       extstate.win_viewport = nil
     end
+    if expected.win_viewport_margins == nil then
+      extstate.win_viewport_margins = nil
+    end
 
     if expected.float_pos then
       expected.float_pos = deepcopy(expected.float_pos)
@@ -993,6 +998,7 @@ function Screen:_handle_grid_destroy(grid)
   if self._options.ext_multigrid then
     self.win_position[grid] = nil
     self.win_viewport[grid] = nil
+    self.win_viewport_margins[grid] = nil
   end
 end
 
@@ -1048,6 +1054,16 @@ function Screen:_handle_win_viewport(
   }
 end
 
+function Screen:_handle_win_viewport_margins(grid, win, top, bottom, left, right)
+  self.win_viewport_margins[grid] = {
+    win = win,
+    top = top,
+    bottom = bottom,
+    left = left,
+    right = right,
+  }
+end
+
 function Screen:_handle_win_float_pos(grid, ...)
   self.win_position[grid] = nil
   self.float_pos[grid] = { ... }
@@ -1466,6 +1482,8 @@ function Screen:_extstate_repr(attr_state)
   end
 
   local win_viewport = (next(self.win_viewport) and self.win_viewport) or nil
+  local win_viewport_margins = (next(self.win_viewport_margins) and self.win_viewport_margins)
+    or nil
 
   return {
     popupmenu = self.popupmenu,
@@ -1480,6 +1498,7 @@ function Screen:_extstate_repr(attr_state)
     msg_history = msg_history,
     float_pos = self.float_pos,
     win_viewport = win_viewport,
+    win_viewport_margins = win_viewport_margins,
   }
 end