vim-patch:8.2.4501: with 'showbreak' set cursor displayed in wrong position

Problem:    With 'showbreak' set and after the end of the line the cursor
            may be displayed in the wrong position.
Solution:   Do not apply 'showbreak' after the end of the line. (closes vim/vim#9884)

https://github.com/vim/vim/commit/21efafe4c25373929979c72dc8aafa119f12dd8b

Co-authored-by: Bram Moolenaar <Bram@vim.org>

3 files changed, 68 insertions, 2 deletions

diff --git a/src/nvim/plines.c b/src/nvim/plines.c
index 42218ac847..bed15f9e36 100644
--- a/src/nvim/plines.c
+++ b/src/nvim/plines.c
@@ -444,9 +444,9 @@ int win_lbr_chartabsize(chartabsize_T *cts, int *headp)
   // May have to add something for 'breakindent' and/or 'showbreak'
   // string at start of line.
   // Set *headp to the size of what we add.
+  // Do not use 'showbreak' at the NUL after the text.
   added = 0;
-
-  char *const sbr = (char *)get_showbreak_value(wp);
+  char *const sbr = c == NUL ? empty_option : (char *)get_showbreak_value(wp);
   if ((*sbr != NUL || wp->w_p_bri) && wp->w_p_wrap && vcol != 0) {
     colnr_T sbrlen = 0;
     int numberwidth = win_col_off(wp);
diff --git a/src/nvim/testdir/test_breakindent.vim b/src/nvim/testdir/test_breakindent.vim
index 8bbac2d237..934dca4793 100644
--- a/src/nvim/testdir/test_breakindent.vim
+++ b/src/nvim/testdir/test_breakindent.vim
@@ -8,6 +8,7 @@ source check.vim
 CheckOption breakindent
 
 source view_util.vim
+source screendump.vim
 
 let s:input ="\tabcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOP"
 
@@ -889,6 +890,27 @@ func Test_window_resize_with_linebreak()
   %bw!
 endfunc
 
+func Test_cursor_position_with_showbreak()
+  CheckScreendump
+
+  let lines =<< trim END
+      vim9script
+      &signcolumn = 'yes'
+      &showbreak = '+ '
+      var leftcol: number = win_getid()->getwininfo()->get(0, {})->get('textoff')
+      repeat('x', &columns - leftcol - 1)->setline(1)
+      'second line'->setline(2)
+  END
+  call writefile(lines, 'XscriptShowbreak')
+  let buf = RunVimInTerminal('-S XscriptShowbreak', #{rows: 6})
+
+  call term_sendkeys(buf, "AX")
+  call VerifyScreenDump(buf, 'Test_cursor_position_with_showbreak', {})
+
+  call StopVimInTerminal(buf)
+  call delete('XscriptShowbreak')
+endfunc
+
 func Test_no_spurious_match()
   let s:input = printf('- y %s y %s', repeat('x', 50), repeat('x', 50))
   call s:test_windows('setl breakindent breakindentopt=list:-1 formatlistpat=^- hls')
diff --git a/test/functional/legacy/breakindent_spec.lua b/test/functional/legacy/breakindent_spec.lua
new file mode 100644
index 0000000000..d7779684a4
--- /dev/null
+++ b/test/functional/legacy/breakindent_spec.lua
@@ -0,0 +1,44 @@
+local helpers = require('test.functional.helpers')(after_each)
+local Screen = require('test.functional.ui.screen')
+local clear = helpers.clear
+local exec = helpers.exec
+local feed = helpers.feed
+
+before_each(clear)
+
+describe('breakindent', function()
+  -- oldtest: Test_cursor_position_with_showbreak()
+  it('cursor shown at correct position with showbreak', function()
+    local screen = Screen.new(75, 6)
+    screen:set_default_attr_ids({
+      [0] = {bold = true, foreground = Screen.colors.Blue},  -- NonText
+      [1] = {background = Screen.colors.Grey, foreground = Screen.colors.DarkBlue},  -- SignColumn
+      [2] = {bold = true},  -- ModeMsg
+    })
+    screen:attach()
+    exec([[
+      let &signcolumn = 'yes'
+      let &showbreak = '+'
+      let leftcol = win_getid()->getwininfo()->get(0, {})->get('textoff')
+      eval repeat('x', &columns - leftcol - 1)->setline(1)
+      eval 'second line'->setline(2)
+    ]])
+    screen:expect([[
+      {1:  }^xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx |
+      {1:  }second line                                                              |
+      {0:~                                                                          }|
+      {0:~                                                                          }|
+      {0:~                                                                          }|
+                                                                                 |
+    ]])
+    feed('AX')
+    screen:expect([[
+      {1:  }xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxX|
+      {1:  }^second line                                                              |
+      {0:~                                                                          }|
+      {0:~                                                                          }|
+      {0:~                                                                          }|
+      {2:-- INSERT --}                                                               |
+    ]])
+  end)
+end)