fix(ui): ensure screen update before waiting for input #30576

Ensure the screen is fully updated before blocking for input. This did
not always happen before, for example when setting `cursorline
scrolloff=9999`, which lead to jerky movement when using some GUI
applications.

Because of the duality of redraw_later, this can't be done in
command-line or when waiting for "Press ENTER". In many of those cases
the redraw is expected AFTER the key press, while normally it should
update the screen immediately. So, those special cases are excluded.

2 files changed, 258 insertions, 3 deletions

diff --git a/src/nvim/state.c b/src/nvim/state.c
index 1b08a08fba..908f724792 100644
--- a/src/nvim/state.c
+++ b/src/nvim/state.c
@@ -66,9 +66,11 @@ getkey:
       // Event was made available after the last multiqueue_process_events call
       key = K_EVENT;
     } else {
-      // Duplicate display updating logic in vgetorpeek()
-      if (((State & MODE_INSERT) != 0 || p_lz) && (State & MODE_CMDLINE) == 0
-          && must_redraw != 0 && !need_wait_return) {
+      // Ensure the screen is fully updated before blocking for input. Because of the duality of
+      // redraw_later, this can't be done in command-line or when waiting for "Press ENTER".
+      // In many of those cases the redraw is expected AFTER the key press, while normally it should
+      // update the screen immediately.
+      if (must_redraw != 0 && !need_wait_return && (State & MODE_CMDLINE) == 0) {
         update_screen();
         setcursor();  // put cursor back where it belongs
       }
diff --git a/test/functional/ui/multigrid_spec.lua b/test/functional/ui/multigrid_spec.lua
index 63ae38d3c3..e009ed0a29 100644
--- a/test/functional/ui/multigrid_spec.lua
+++ b/test/functional/ui/multigrid_spec.lua
@@ -2599,4 +2599,257 @@ describe('ext_multigrid', function()
     ]])
     eq(1, api.nvim_get_option_value('cmdheight', {}))
   end)
+
+  describe('centered cursorline', function()
+    before_each(function()
+      -- Force a centered cursorline, this caused some redrawing problems described in #30576.
+      -- Most importantly, win_viewport was not received in time, and sum_scroll_delta did not refresh.
+      command('set cursorline scrolloff=9999')
+    end)
+    it('insert line scrolls correctly', function()
+      for i = 1, 11 do
+        insert('line' .. i .. '\n')
+      end
+      screen:expect({
+        grid = [[
+        ## grid 1
+          [2:-----------------------------------------------------]|*12
+          {11:[No Name] [+]                                        }|
+          [3:-----------------------------------------------------]|
+        ## grid 2
+          line1                                                |
+          line2                                                |
+          line3                                                |
+          line4                                                |
+          line5                                                |
+          line6                                                |
+          line7                                                |
+          line8                                                |
+          line9                                                |
+          line10                                               |
+          line11                                               |
+          {23:^                                                     }|
+        ## grid 3
+                                                               |
+        ]], win_viewport={
+        [2] = {win = 1000, topline = 0, botline = 12, curline = 11, curcol = 0, linecount = 12, sum_scroll_delta = 0};
+      }, win_viewport_margins={
+        [2] = {
+          bottom = 0,
+          left = 0,
+          right = 0,
+          top = 0,
+          win = 1000
+        }
+      }})
+      insert('line12\n')
+      screen:expect({
+        grid = [[
+        ## grid 1
+          [2:-----------------------------------------------------]|*12
+          {11:[No Name] [+]                                        }|
+          [3:-----------------------------------------------------]|
+        ## grid 2
+          line2                                                |
+          line3                                                |
+          line4                                                |
+          line5                                                |
+          line6                                                |
+          line7                                                |
+          line8                                                |
+          line9                                                |
+          line10                                               |
+          line11                                               |
+          line12                                               |
+          {23:^                                                     }|
+        ## grid 3
+                                                               |
+        ]], win_viewport={
+        [2] = {win = 1000, topline = 1, botline = 13, curline = 12, curcol = 0, linecount = 13, sum_scroll_delta = 1};
+      }, win_viewport_margins={
+        [2] = {
+          bottom = 0,
+          left = 0,
+          right = 0,
+          top = 0,
+          win = 1000
+        }
+      }})
+    end)
+
+    it('got to top scrolls correctly', function()
+      for i = 1, 20 do
+        insert('line' .. i .. '\n')
+      end
+      screen:expect({
+        grid = [[
+        ## grid 1
+          [2:-----------------------------------------------------]|*12
+          {11:[No Name] [+]                                        }|
+          [3:-----------------------------------------------------]|
+        ## grid 2
+          line10                                               |
+          line11                                               |
+          line12                                               |
+          line13                                               |
+          line14                                               |
+          line15                                               |
+          line16                                               |
+          line17                                               |
+          line18                                               |
+          line19                                               |
+          line20                                               |
+          {23:^                                                     }|
+        ## grid 3
+                                                               |
+        ]], win_viewport={
+        [2] = {win = 1000, topline = 9, botline = 21, curline = 20, curcol = 0, linecount = 21, sum_scroll_delta = 9};
+      }, win_viewport_margins={
+        [2] = {
+          bottom = 0,
+          left = 0,
+          right = 0,
+          top = 0,
+          win = 1000
+        }
+      }})
+      feed('gg')
+      screen:expect({
+        grid = [[
+        ## grid 1
+          [2:-----------------------------------------------------]|*12
+          {11:[No Name] [+]                                        }|
+          [3:-----------------------------------------------------]|
+        ## grid 2
+          {23:^line1                                                }|
+          line2                                                |
+          line3                                                |
+          line4                                                |
+          line5                                                |
+          line6                                                |
+          line7                                                |
+          line8                                                |
+          line9                                                |
+          line10                                               |
+          line11                                               |
+          line12                                               |
+        ## grid 3
+                                                               |
+        ]], win_viewport={
+        [2] = {win = 1000, topline = 0, botline = 13, curline = 0, curcol = 0, linecount = 21, sum_scroll_delta = 0};
+      }, win_viewport_margins={
+        [2] = {
+          bottom = 0,
+          left = 0,
+          right = 0,
+          top = 0,
+          win = 1000
+        }
+      }})
+    end)
+
+    it('scrolls in the middle', function()
+      for i = 1, 20 do
+        insert('line' .. i .. '\n')
+      end
+      screen:expect({
+        grid = [[
+        ## grid 1
+          [2:-----------------------------------------------------]|*12
+          {11:[No Name] [+]                                        }|
+          [3:-----------------------------------------------------]|
+        ## grid 2
+          line10                                               |
+          line11                                               |
+          line12                                               |
+          line13                                               |
+          line14                                               |
+          line15                                               |
+          line16                                               |
+          line17                                               |
+          line18                                               |
+          line19                                               |
+          line20                                               |
+          {23:^                                                     }|
+        ## grid 3
+                                                               |
+        ]], win_viewport={
+        [2] = {win = 1000, topline = 9, botline = 21, curline = 20, curcol = 0, linecount = 21, sum_scroll_delta = 9};
+      }, win_viewport_margins={
+        [2] = {
+          bottom = 0,
+          left = 0,
+          right = 0,
+          top = 0,
+          win = 1000
+        }
+      }})
+      feed('M')
+      screen:expect({
+        grid = [[
+        ## grid 1
+          [2:-----------------------------------------------------]|*12
+          {11:[No Name] [+]                                        }|
+          [3:-----------------------------------------------------]|
+        ## grid 2
+          line10                                               |
+          line11                                               |
+          line12                                               |
+          line13                                               |
+          line14                                               |
+          {23:^line15                                               }|
+          line16                                               |
+          line17                                               |
+          line18                                               |
+          line19                                               |
+          line20                                               |
+                                                               |
+        ## grid 3
+                                                               |
+        ]], win_viewport={
+        [2] = {win = 1000, topline = 9, botline = 21, curline = 14, curcol = 0, linecount = 21, sum_scroll_delta = 9};
+      }, win_viewport_margins={
+        [2] = {
+          bottom = 0,
+          left = 0,
+          right = 0,
+          top = 0,
+          win = 1000
+        }
+      }})
+      feed('k')
+      screen:expect({
+        grid = [[
+        ## grid 1
+          [2:-----------------------------------------------------]|*12
+          {11:[No Name] [+]                                        }|
+          [3:-----------------------------------------------------]|
+        ## grid 2
+          line9                                                |
+          line10                                               |
+          line11                                               |
+          line12                                               |
+          line13                                               |
+          {23:^line14                                               }|
+          line15                                               |
+          line16                                               |
+          line17                                               |
+          line18                                               |
+          line19                                               |
+          line20                                               |
+        ## grid 3
+                                                               |
+        ]], win_viewport={
+        [2] = {win = 1000, topline = 8, botline = 21, curline = 13, curcol = 0, linecount = 21, sum_scroll_delta = 8};
+      }, win_viewport_margins={
+        [2] = {
+          bottom = 0,
+          left = 0,
+          right = 0,
+          top = 0,
+          win = 1000
+        }
+      }})
+    end)
+  end)
 end)