fix(lsp): treat 2-triggers-at-once as "last char wins" #35435

Problem:
If there are 2 language servers with different trigger chars (`-` and
`>`), and a keymap inputs both simultaneously (`->`), then `>` doesn't
trigger. We get completion items from server1 only.

This happens because the `completion_timer` for the `-` trigger is still
pending.

Solution:
If the next character arrived enough quickly (< 25 ms), replace the
existing deferred autotrigger with a new one that matches this later
character.

(cherry picked from commit e69b81ad94)

runtime/lua/vim/lsp/completion.lua

@@ -571,7 +571,10 @@ local function on_insert_char_pre(handle)
 
   local char = api.nvim_get_vvar('char')
   local matched_clients = handle.triggers[char]
-  if not completion_timer and matched_clients then
+  -- Discard pending trigger char, complete the "latest" one.
+  -- Can happen if a mapping inputs multiple trigger chars simultaneously.
+  reset_timer()
+  if matched_clients then
     completion_timer = assert(vim.uv.new_timer())
     completion_timer:start(25, 0, function()
       reset_timer()

test/functional/plugin/lsp/completion_spec.lua

@@ -1037,6 +1037,34 @@ describe('vim.lsp.completion: protocol', function()
     end)
   end)
 
+  it('treats 2-triggers-at-once as "last char wins"', function()
+    local results1 = {
+      isIncomplete = false,
+      items = {
+        {
+          label = 'first',
+        },
+      },
+    }
+    create_server('dummy1', results1, { trigger_chars = { '-' } })
+    local results2 = {
+      isIncomplete = false,
+      items = {
+        {
+          label = 'second',
+        },
+      },
+    }
+    create_server('dummy2', results2, { trigger_chars = { '>' } })
+
+    feed('i->')
+
+    assert_matches(function(matches)
+      eq(1, #matches)
+      eq('second', matches[1].word)
+    end)
+  end)
+
   it('executes commands', function()
     local completion_list = {
       isIncomplete = false,