vim-patch:8.2.4718: @@@ in the last line sometimes drawn in the wrong place (#18055)

Problem:    @@@ in the last line sometimes drawn in the wrong place.
Solution:   Make sure the column is valid. (closes vim/vim#10130)
https://github.com/vim/vim/commit/cee9c844f27bceaba90362a3fa27a04d4d06c0fd

3 files changed, 83 insertions, 12 deletions

diff --git a/src/nvim/screen.c b/src/nvim/screen.c
index baeb2fcb03..296255ed8c 100644
--- a/src/nvim/screen.c
+++ b/src/nvim/screen.c
@@ -1639,16 +1639,18 @@ static void win_update(win_T *wp, DecorProviders *providers)
       int scr_row = wp->w_grid.Rows - 1;
 
       // Last line isn't finished: Display "@@@" in the last screen line.
-      grid_puts_len(&wp->w_grid, (char_u *)"@@", 2, scr_row, 0, at_attr);
+      grid_puts_len(&wp->w_grid, (char_u *)"@@", MIN(wp->w_grid.Columns, 2), scr_row, 0, at_attr);
 
       grid_fill(&wp->w_grid, scr_row, scr_row + 1, 2, wp->w_grid.Columns,
                 '@', ' ', at_attr);
       set_empty_rows(wp, srow);
       wp->w_botline = lnum;
     } else if (dy_flags & DY_LASTLINE) {      // 'display' has "lastline"
+      int start_col = wp->w_grid.Columns - 3;
+
       // Last line isn't finished: Display "@@@" at the end.
       grid_fill(&wp->w_grid, wp->w_grid.Rows - 1, wp->w_grid.Rows,
-                wp->w_grid.Columns - 3, wp->w_grid.Columns, '@', '@', at_attr);
+                MAX(start_col, 0), wp->w_grid.Columns, '@', '@', at_attr);
       set_empty_rows(wp, srow);
       wp->w_botline = lnum;
     } else {
@@ -6001,9 +6003,9 @@ static void end_search_hl(void)
 }
 
 
-/// Fill the grid from 'start_row' to 'end_row', from 'start_col' to 'end_col'
-/// with character 'c1' in first column followed by 'c2' in the other columns.
-/// Use attributes 'attr'.
+/// Fill the grid from "start_row" to "end_row" (exclusive), from "start_col"
+/// to "end_col" (exclusive) with character "c1" in first column followed by
+/// "c2" in the other columns.  Use attributes "attr".
 void grid_fill(ScreenGrid *grid, int start_row, int end_row, int start_col, int end_col, int c1,
                int c2, int attr)
 {
diff --git a/src/nvim/testdir/test_display.vim b/src/nvim/testdir/test_display.vim
index 094283a3a3..6938abbc28 100644
--- a/src/nvim/testdir/test_display.vim
+++ b/src/nvim/testdir/test_display.vim
@@ -325,4 +325,31 @@ func Test_display_linebreak_breakat()
   let &breakat=_breakat
 endfunc
 
+func Test_display_lastline()
+  CheckScreendump
+
+  let lines =<< trim END
+      call setline(1, ['aaa', 'b'->repeat(100)])
+      set display=truncate
+      vsplit
+      100wincmd <
+  END
+  call writefile(lines, 'XdispLastline')
+  let buf = RunVimInTerminal('-S XdispLastline', #{rows: 10})
+  call VerifyScreenDump(buf, 'Test_display_lastline_1', {})
+
+  call term_sendkeys(buf, ":set display=lastline\<CR>")
+  call VerifyScreenDump(buf, 'Test_display_lastline_2', {})
+
+  call term_sendkeys(buf, ":100wincmd >\<CR>")
+  call VerifyScreenDump(buf, 'Test_display_lastline_3', {})
+
+  call term_sendkeys(buf, ":set display=truncate\<CR>")
+  call VerifyScreenDump(buf, 'Test_display_lastline_4', {})
+
+  call StopVimInTerminal(buf)
+  call delete('XdispLastline')
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab
diff --git a/test/functional/legacy/display_spec.lua b/test/functional/legacy/display_spec.lua
index 59ba170e33..4c7915403c 100644
--- a/test/functional/legacy/display_spec.lua
+++ b/test/functional/legacy/display_spec.lua
@@ -2,23 +2,21 @@ local helpers = require('test.functional.helpers')(after_each)
 
 local Screen = require('test.functional.ui.screen')
 local clear = helpers.clear
-local poke_eventloop = helpers.poke_eventloop
 local exec = helpers.exec
 local feed = helpers.feed
-local feed_command = helpers.feed_command
+local command = helpers.command
 
 describe('display', function()
   before_each(clear)
 
-  it('scroll when modified at topline', function()
+  it('scroll when modified at topline vim-patch:8.2.1488', function()
     local screen = Screen.new(20, 4)
     screen:attach()
     screen:set_default_attr_ids({
       [1] = {bold = true},
     })
 
-    feed_command([[call setline(1, repeat('a', 21))]])
-    poke_eventloop()
+    command([[call setline(1, repeat('a', 21))]])
     feed('O')
     screen:expect([[
       ^                    |
@@ -28,7 +26,7 @@ describe('display', function()
     ]])
   end)
 
-  it('scrolling when modified at topline in Visual mode', function()
+  it('scrolling when modified at topline in Visual mode vim-patch:8.2.4626', function()
     local screen = Screen.new(60, 8)
     screen:attach()
     screen:set_default_attr_ids({
@@ -57,5 +55,49 @@ describe('display', function()
       {1:-- VISUAL LINE --}                                           |
     ]])
   end)
-end)
 
+  it('@@@ in the last line shows correctly in a narrow window vim-patch:8.2.4718', function()
+    local screen = Screen.new(60, 10)
+    screen:set_default_attr_ids({
+      [1] = {bold = true, foreground = Screen.colors.Blue},  -- NonText
+      [2] = {bold = true, reverse = true},  -- StatusLine
+      [3] = {reverse = true},  -- VertSplit, StatusLineNC
+    })
+    screen:attach()
+    exec([[
+      call setline(1, ['aaa', 'b'->repeat(100)])
+      set display=truncate
+      vsplit
+      100wincmd <
+    ]])
+    screen:expect([[
+      ^a{3:│}aaa                                                       |
+      a{3:│}bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb|
+      a{3:│}bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb                |
+      b{3:│}{1:~                                                         }|
+      b{3:│}{1:~                                                         }|
+      b{3:│}{1:~                                                         }|
+      b{3:│}{1:~                                                         }|
+      {1:@}{3:│}{1:~                                                         }|
+      {2:< }{3:[No Name] [+]                                             }|
+                                                                  |
+    ]])
+    command('set display=lastline')
+    screen:expect_unchanged()
+    command('100wincmd >')
+    screen:expect([[
+      ^aaa                                                       {3:│}a|
+      bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb{3:│}a|
+      bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb                {3:│}a|
+      {1:~                                                         }{3:│}b|
+      {1:~                                                         }{3:│}b|
+      {1:~                                                         }{3:│}b|
+      {1:~                                                         }{3:│}b|
+      {1:~                                                         }{3:│}{1:@}|
+      {2:[No Name] [+]                                              }{3:<}|
+                                                                  |
+    ]])
+    command('set display=truncate')
+    screen:expect_unchanged()
+  end)
+end)