[release-0.4] win_update: fix redraw regression (#11027)

Before 6e9ea5adc `win_ins_lines` would return `FAIL` for `i/line_count == 0`.

Handle this by checking it in the outer `if`.

Ref: https://github.com/neovim/neovim/commit/6e9ea5ad#commitcomment-35084669

src/nvim/screen.c

@@ -871,7 +871,7 @@ static void win_update(win_T *wp)
         if (wp->w_lines[0].wl_lnum != wp->w_topline)
           i += diff_check_fill(wp, wp->w_lines[0].wl_lnum)
                - wp->w_old_topfill;
-        if (i < wp->w_grid.Rows - 2) {  // less than a screen off
+        if (i != 0 && i < wp->w_grid.Rows - 2) {  // less than a screen off
           // Try to insert the correct number of lines.
           // If not the last window, delete the lines at the bottom.
           // win_ins_lines may fail when the terminal can't do it.

test/functional/ui/diff_spec.lua

@@ -3,6 +3,8 @@ local Screen = require('test.functional.ui.screen')
 
 local feed = helpers.feed
 local clear = helpers.clear
+local command = helpers.command
+local insert = helpers.insert
 local write_file = helpers.write_file
 
 describe('Diff mode screen', function()
@@ -957,3 +959,75 @@ int main(int argc, char **argv)
     end)
   end)
 end)
+
+it('win_update redraws lines properly', function()
+  local screen
+  clear()
+  screen = Screen.new(30, 10)
+  screen:attach()
+  screen:set_default_attr_ids({
+    [1] = {bold = true, foreground = Screen.colors.Blue1},
+    [2] = {foreground = Screen.colors.Grey100, background = Screen.colors.Red},
+    [3] = {background = Screen.colors.Red, foreground = Screen.colors.Grey100, special = Screen.colors.Yellow},
+    [4] = {bold = true, foreground = Screen.colors.SeaGreen4},
+    [5] = {special = Screen.colors.Yellow},
+    [6] = {special = Screen.colors.Yellow, bold = true, foreground = Screen.colors.SeaGreen4},
+    [7] = {foreground = Screen.colors.Grey0, background = Screen.colors.Grey100},
+    [8] = {foreground = Screen.colors.Gray90, background = Screen.colors.Grey100},
+    [9] = {foreground = tonumber('0x00000c'), background = Screen.colors.Grey100},
+    [10] = {background = Screen.colors.Grey100, bold = true, foreground = tonumber('0xe5e5ff')},
+    [11] = {background = Screen.colors.Grey100, bold = true, foreground = tonumber('0x2b8452')},
+    [12] = {bold = true, reverse = true},
+    [13] = {foreground = Screen.colors.DarkBlue, background = Screen.colors.WebGray},
+    [14] = {reverse = true},
+    [15] = {background = Screen.colors.LightBlue},
+    [16] = {background = Screen.colors.LightCyan1, bold = true, foreground = Screen.colors.Blue1},
+    [17] = {bold = true, background = Screen.colors.Red},
+    [18] = {background = Screen.colors.LightMagenta},
+  })
+
+  insert([[
+  1
+
+
+  2
+  1a
+  ]])
+  command("vnew")
+  insert([[
+  2
+  2a
+  2b
+  ]])
+  command("windo diffthis")
+  command("windo 1")
+  screen:expect{grid=[[
+    {13:  }{16:-------}{14:│}{13:  }{15:^1                 }|
+    {13:  }{16:-------}{14:│}{13:  }{15:                  }|
+    {13:  }{16:-------}{14:│}{13:  }{15:                  }|
+    {13:  }2      {14:│}{13:  }2                 |
+    {13:  }{17:2}{18:a     }{14:│}{13:  }{17:1}{18:a                }|
+    {13:  }{15:2b     }{14:│}{13:  }{16:------------------}|
+    {13:  }       {14:│}{13:  }                  |
+    {1:~        }{14:│}{1:~                   }|
+    {14:<me] [+]  }{12:[No Name] [+]       }|
+                                  |
+  ]]}
+  feed('<C-e>')
+  feed('<C-e>')
+  feed('<C-y>')
+  feed('<C-y>')
+  feed('<C-y>')
+  screen:expect{grid=[[
+    {13:  }{16:-------}{14:│}{13:  }{15:1                 }|
+    {13:  }{16:-------}{14:│}{13:  }{15:                  }|
+    {13:  }{16:-------}{14:│}{13:  }{15:^                  }|
+    {13:  }2      {14:│}{13:  }2                 |
+    {13:  }{17:2}{18:a     }{14:│}{13:  }{17:1}{18:a                }|
+    {13:  }{15:2b     }{14:│}{13:  }{16:------------------}|
+    {13:  }       {14:│}{13:  }                  |
+    {1:~        }{14:│}{1:~                   }|
+    {14:<me] [+]  }{12:[No Name] [+]       }|
+                                  |
+  ]]}
+end)