feat: config.view.entries.vertical_positioning = 'auto'|'above'|'below'

'above' works best with vim.opt.scrolloff = 999.

Fixes: hrsh7th#495
Helped-by: Thanatermesis <thanatermesis@gmail.com>

lua/cmp/view/custom_entries_view.lua

@@ -172,12 +172,25 @@ custom_entries_view.open = function(self, offset, entries)
   local border_info = window.get_border_info({ style = completion })
   local border_offset_row = border_info.top + border_info.bottom
   local border_offset_col = border_info.left + border_info.right
-  if math.floor(vim.o.lines * 0.5) <= row + border_offset_row and vim.o.lines - row - border_offset_row <= math.min(DEFAULT_HEIGHT, height) then
+
+  local prefers_above = c.view.entries.vertical_positioning == 'above'
+  local prefers_auto = c.view.entries.vertical_positioning == 'auto'
+  local cant_fit_at_bottom = vim.o.lines - row - border_offset_row <= math.min(DEFAULT_HEIGHT, height)
+  local cant_fit_at_top = row - border_offset_row <= math.min(DEFAULT_HEIGHT, height)
+  local is_in_top_half = math.floor(vim.o.lines * 0.5) > row + border_offset_row
+  local should_position_above =
+    cant_fit_at_bottom or
+    (prefers_above and not cant_fit_at_top) or
+    (prefers_auto and is_in_top_half)
+  if should_position_above then
+    self.bottom_up = true
     height = math.min(height, row - 1)
     row = row - height - border_offset_row - 1
     if row < 0 then
       height = height + row
     end
+  else
+    self.bottom_up = false
   end
   if math.floor(vim.o.columns * 0.5) <= col + border_offset_col and vim.o.columns - col - border_offset_col <= width then
     width = math.min(width, vim.o.columns - 1)
@@ -187,12 +200,6 @@ custom_entries_view.open = function(self, offset, entries)
     end
   end
 
-  if pos[1] > row then
-    self.bottom_up = true
-  else
-    self.bottom_up = false
-  end
-
   if not self:is_direction_top_down() then
     local n = #self.entries
     for i = 1, math.floor(n / 2) do
@@ -226,9 +233,9 @@ custom_entries_view.open = function(self, offset, entries)
 
   -- Always set cursor when starting. It will be adjusted on the call to _select
   vim.api.nvim_win_set_cursor(self.entries_win.win, { 1, 0 })
-  if preselect_index > 0 and config.get().preselect == types.cmp.PreselectMode.Item then
+  if preselect_index > 0 and c.preselect == types.cmp.PreselectMode.Item then
     self:_select(preselect_index, { behavior = types.cmp.SelectBehavior.Select, active = false })
-  elseif not string.match(config.get().completion.completeopt, 'noselect') then
+  elseif not string.match(c.completion.completeopt, 'noselect') then
     if self:is_direction_top_down() then
       self:_select(1, { behavior = types.cmp.SelectBehavior.Select, active = false })
     else