vim-patch:8.1.1071: cannot get composing characters from the screen

Problem:    Cannot get composing characters from the screen.
Solution:   Add screenchars() and screenstring(). (partly by Ozaki Kiichi,
            closes vim/vim#4059)
https://github.com/vim/vim/commit/2912abb3a2fd72074e3901c8ae1d4a77ce764675

4 files changed, 104 insertions, 17 deletions

diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index 5f355abe3c..c0a18b3236 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -290,9 +290,11 @@ return {
     rubyeval={args=1},
     screenattr={args=2},
     screenchar={args=2},
+    screenchars={args=2},
     screencol={},
     screenpos={args=3},
     screenrow={},
+    screenstring={args=2},
     search={args={1, 4}},
     searchcount={args={0,1}},
     searchdecl={args={1, 3}},
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 35ebd0f3f8..a71e33dada 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -8108,9 +8108,7 @@ static void f_rpcstop(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   }
 }
 
-/*
- * "screenattr()" function
- */
+// "screenattr()" function
 static void f_screenattr(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
   int c;
@@ -8128,9 +8126,7 @@ static void f_screenattr(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   rettv->vval.v_number = c;
 }
 
-/*
- * "screenchar()" function
- */
+// "screenchar()" function
 static void f_screenchar(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
   int c;
@@ -8148,11 +8144,34 @@ static void f_screenchar(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   rettv->vval.v_number = c;
 }
 
-/*
- * "screencol()" function
- *
- * First column is 1 to be consistent with virtcol().
- */
+// "screenchars()" function
+static void f_screenchars(typval_T *argvars, typval_T *rettv, FunPtr fptr)
+{
+  int row = tv_get_number_chk(&argvars[0], NULL) - 1;
+  int col = tv_get_number_chk(&argvars[1], NULL) - 1;
+  if (row < 0 || row >= default_grid.Rows
+      || col < 0 || col >= default_grid.Columns) {
+    tv_list_alloc_ret(rettv, 0);
+    return;
+  }
+  ScreenGrid *grid = &default_grid;
+  screenchar_adjust_grid(&grid, &row, &col);
+  int pcc[MAX_MCO];
+  int c = utfc_ptr2char(grid->chars[grid->line_offset[row] + col], pcc);
+  int composing_len = 0;
+  while (pcc[composing_len] != 0) {
+    composing_len++;
+  }
+  tv_list_alloc_ret(rettv, composing_len + 1);
+  tv_list_append_number(rettv->vval.v_list, c);
+  for (int i = 0; i < composing_len; i++) {
+    tv_list_append_number(rettv->vval.v_list, pcc[i]);
+  }
+}
+
+// "screencol()" function
+//
+// First column is 1 to be consistent with virtcol().
 static void f_screencol(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
   rettv->vval.v_number = ui_current_col() + 1;
@@ -8184,17 +8203,29 @@ static void f_screenpos(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   tv_dict_add_nr(dict, S_LEN("endcol"), ecol);
 }
 
-/*
- * "screenrow()" function
- */
+// "screenrow()" function
 static void f_screenrow(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
   rettv->vval.v_number = ui_current_row() + 1;
 }
 
-/*
- * "search()" function
- */
+// "screenstring()" function
+static void f_screenstring(typval_T *argvars, typval_T *rettv, FunPtr fptr)
+{
+  rettv->vval.v_string = NULL;
+  rettv->v_type = VAR_STRING;
+  int row = tv_get_number_chk(&argvars[0], NULL) - 1;
+  int col = tv_get_number_chk(&argvars[1], NULL) - 1;
+  if (row < 0 || row >= default_grid.Rows
+      || col < 0 || col >= default_grid.Columns) {
+    return;
+  }
+  ScreenGrid *grid = &default_grid;
+  screenchar_adjust_grid(&grid, &row, &col);
+  rettv->vval.v_string = vim_strsave(grid->chars[grid->line_offset[row] + col]);
+}
+
+// "search()" function
 static void f_search(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
   int flags = 0;
diff --git a/src/nvim/testdir/test_utf8.vim b/src/nvim/testdir/test_utf8.vim
index c51fb3a759..acf15e0e28 100644
--- a/src/nvim/testdir/test_utf8.vim
+++ b/src/nvim/testdir/test_utf8.vim
@@ -1,5 +1,6 @@
 " Tests for Unicode manipulations
  
+source view_util.vim
 
 " Visual block Insert adjusts for multi-byte char
 func Test_visual_block_insert()
@@ -61,6 +62,40 @@ func Test_getvcol()
   call assert_equal(2, virtcol("']"))
 endfunc
 
+func Test_screenchar_utf8()
+  new
+
+  " 1-cell, with composing characters 
+  call setline(1, ["ABC\u0308"])
+  redraw
+  call assert_equal([0x0041], screenchars(1, 1))
+  call assert_equal([0x0042], screenchars(1, 2))
+  call assert_equal([0x0043, 0x0308], screenchars(1, 3))
+  call assert_equal("A", screenstring(1, 1))
+  call assert_equal("B", screenstring(1, 2))
+  call assert_equal("C\u0308", screenstring(1, 3))
+
+  " 2-cells, with composing characters 
+  let text = "\u3042\u3044\u3046\u3099"
+  call setline(1, text)
+  redraw
+  call assert_equal([0x3042], screenchars(1, 1))
+  call assert_equal([0], screenchars(1, 2))
+  call assert_equal([0x3044], screenchars(1, 3))
+  call assert_equal([0], screenchars(1, 4))
+  call assert_equal([0x3046, 0x3099], screenchars(1, 5))
+
+  call assert_equal("\u3042", screenstring(1, 1))
+  call assert_equal("", screenstring(1, 2))
+  call assert_equal("\u3044", screenstring(1, 3))
+  call assert_equal("", screenstring(1, 4))
+  call assert_equal("\u3046\u3099", screenstring(1, 5))
+
+  call assert_equal([text . '  '], ScreenLinesUtf8(1, 8))
+
+  bwipe!
+endfunc
+
 func Test_list2str_str2list_utf8()
   " One Unicode codepoint
   let s = "\u3042\u3044"
diff --git a/src/nvim/testdir/view_util.vim b/src/nvim/testdir/view_util.vim
index 1def201a05..dd143c02c1 100644
--- a/src/nvim/testdir/view_util.vim
+++ b/src/nvim/testdir/view_util.vim
@@ -34,6 +34,25 @@ function! ScreenLines(lnum, width) abort
   return lines
 endfunction
 
+" Get text on the screen, including composing characters.
+" ScreenLines(lnum, width) or
+" ScreenLines([start, end], width)
+function! ScreenLinesUtf8(lnum, width) abort
+  redraw!
+  if type(a:lnum) == v:t_list
+    let start = a:lnum[0]
+    let end = a:lnum[1]
+  else
+    let start = a:lnum
+    let end = a:lnum
+  endif
+  let lines = []
+  for l in range(start, end)
+    let lines += [join(map(range(1, a:width), 'screenstring(l, v:val)'), '')]
+  endfor
+  return lines
+endfunction
+
 function! ScreenAttrs(lnum, width) abort
   redraw!
   if type(a:lnum) == v:t_list