fix(pum_redraw): use grid_puts_len() to truncate the text

Nvim already resizes grid to the required width, so there is no need to
truncate the text in pum_redraw(). What's more, truncation is currently
done incorrectly because Vim patch 8.2.1995 was ported incorrectly.

This nearly reverts the truncation part of Vim patch 8.2.1995, but not
the part that reduces unnecessary calls to pum_redraw(). The original PR
https://github.com/vim/vim/pull/7306 didn't explain much about which
part of it actually reduces redraws.

2 files changed, 132 insertions, 11 deletions

diff --git a/src/nvim/popupmnu.c b/src/nvim/popupmnu.c
index 606c03f838..b253077844 100644
--- a/src/nvim/popupmnu.c
+++ b/src/nvim/popupmnu.c
@@ -542,17 +542,8 @@ void pum_redraw(void)
               xfree(st);
               col -= width;
             } else {
-              int size = (int)STRLEN(st);
-              int cells = (int)mb_string2cells(st);
-
-              // only draw the text that fits
-              while (size > 0 && col + cells > pum_width + pum_col) {
-                size--;
-                size -= utf_head_off(st, st + size);
-                cells -= utf_ptr2cells(st + size);
-              }
-
-              grid_puts_len(&pum_grid, st, size, row, col, attr);
+              // use grid_puts_len() to truncate the text
+              grid_puts(&pum_grid, st, row, col, attr);
               xfree(st);
               col += width;
             }
diff --git a/test/functional/ui/popupmenu_spec.lua b/test/functional/ui/popupmenu_spec.lua
index aeba049557..4fc5c389e5 100644
--- a/test/functional/ui/popupmenu_spec.lua
+++ b/test/functional/ui/popupmenu_spec.lua
@@ -2213,4 +2213,134 @@ describe('builtin popupmenu', function()
     feed('<c-y>')
     assert_alive()
   end)
+
+  it('truncates double-width character correctly when there is no scrollbar', function()
+    screen:try_resize(32,8)
+    command('set completeopt+=menuone,noselect')
+    feed('i' .. string.rep(' ', 13))
+    funcs.complete(14, {'哦哦哦哦哦哦哦哦哦哦'})
+    screen:expect([[
+                   ^                   |
+      {1:~           }{n: 哦哦哦哦哦哦哦哦哦>}|
+      {1:~                               }|
+      {1:~                               }|
+      {1:~                               }|
+      {1:~                               }|
+      {1:~                               }|
+      {2:-- INSERT --}                    |
+    ]])
+  end)
+
+  it('truncates double-width character correctly when there is scrollbar', function()
+    screen:try_resize(32,8)
+    command('set completeopt+=noselect')
+    command('set pumheight=4')
+    feed('i' .. string.rep(' ', 12))
+    local items = {}
+    for _ = 1, 8 do
+      table.insert(items, {word = '哦哦哦哦哦哦哦哦哦哦', equal = 1, dup = 1})
+    end
+    funcs.complete(13, items)
+    screen:expect([[
+                  ^                    |
+      {1:~          }{n: 哦哦哦哦哦哦哦哦哦>}{c: }|
+      {1:~          }{n: 哦哦哦哦哦哦哦哦哦>}{c: }|
+      {1:~          }{n: 哦哦哦哦哦哦哦哦哦>}{s: }|
+      {1:~          }{n: 哦哦哦哦哦哦哦哦哦>}{s: }|
+      {1:~                               }|
+      {1:~                               }|
+      {2:-- INSERT --}                    |
+    ]])
+  end)
+end)
+
+describe('builtin popupmenu with ui/ext_multigrid', function()
+  local screen
+  before_each(function()
+    clear()
+    screen = Screen.new(32, 20)
+    screen:attach({ext_multigrid=true})
+    screen:set_default_attr_ids({
+      -- popup selected item / scrollbar track
+      ['s'] = {background = Screen.colors.WebGray},
+      -- popup non-selected item
+      ['n'] = {background = Screen.colors.LightMagenta},
+      -- popup scrollbar knob
+      ['c'] = {background = Screen.colors.Grey0},
+      [1] = {bold = true, foreground = Screen.colors.Blue},
+      [2] = {bold = true},
+      [3] = {reverse = true},
+      [4] = {bold = true, reverse = true},
+      [5] = {bold = true, foreground = Screen.colors.SeaGreen},
+      [6] = {foreground = Screen.colors.Grey100, background = Screen.colors.Red},
+    })
+  end)
+
+  it('truncates double-width character correctly when there is no scrollbar', function()
+    screen:try_resize(32,8)
+    command('set completeopt+=menuone,noselect')
+    feed('i' .. string.rep(' ', 13))
+    funcs.complete(14, {'哦哦哦哦哦哦哦哦哦哦'})
+    screen:expect({grid=[[
+      ## grid 1
+        [2:--------------------------------]|
+        [2:--------------------------------]|
+        [2:--------------------------------]|
+        [2:--------------------------------]|
+        [2:--------------------------------]|
+        [2:--------------------------------]|
+        [2:--------------------------------]|
+        [3:--------------------------------]|
+      ## grid 2
+                     ^                   |
+        {1:~                               }|
+        {1:~                               }|
+        {1:~                               }|
+        {1:~                               }|
+        {1:~                               }|
+        {1:~                               }|
+      ## grid 3
+        {2:-- INSERT --}                    |
+      ## grid 4
+        {n: 哦哦哦哦哦哦哦哦哦>}|
+    ]], float_pos={[4] = {{id = -1}, 'NW', 2, 1, 12, false, 100}}})
+  end)
+
+  it('truncates double-width character correctly when there is scrollbar', function()
+    screen:try_resize(32,8)
+    command('set completeopt+=noselect')
+    command('set pumheight=4')
+    feed('i' .. string.rep(' ', 12))
+    local items = {}
+    for _ = 1, 8 do
+      table.insert(items, {word = '哦哦哦哦哦哦哦哦哦哦', equal = 1, dup = 1})
+    end
+    funcs.complete(13, items)
+    screen:expect({grid=[[
+      ## grid 1
+        [2:--------------------------------]|
+        [2:--------------------------------]|
+        [2:--------------------------------]|
+        [2:--------------------------------]|
+        [2:--------------------------------]|
+        [2:--------------------------------]|
+        [2:--------------------------------]|
+        [3:--------------------------------]|
+      ## grid 2
+                    ^                    |
+        {1:~                               }|
+        {1:~                               }|
+        {1:~                               }|
+        {1:~                               }|
+        {1:~                               }|
+        {1:~                               }|
+      ## grid 3
+        {2:-- INSERT --}                    |
+      ## grid 4
+        {n: 哦哦哦哦哦哦哦哦哦>}{c: }|
+        {n: 哦哦哦哦哦哦哦哦哦>}{c: }|
+        {n: 哦哦哦哦哦哦哦哦哦>}{s: }|
+        {n: 哦哦哦哦哦哦哦哦哦>}{s: }|
+    ]], float_pos={[4] = {{id = -1}, 'NW', 2, 1, 11, false, 100}}})
+  end)
 end)