fix(lsp): preserve trigger chars on completion #39850

Problem:
After 767fbd8, typing trigger chars would open completion but the
chars were removed.

Solution:
Use filterText fallback so selected item respects typed trigger chars.

runtime/lua/vim/lsp/completion.lua

@@ -168,6 +168,13 @@ local function apply_snippet(item)
   end
 end
 
+local function fallback_filtertext(item, word, prefix, match)
+  if item.filterText and not match(word, prefix) and match(item.filterText, prefix) then
+    return item.filterText
+  end
+  return word
+end
+
 --- Returns text that should be inserted when a selecting completion item. The
 --- precedence is as follows: textEdit.newText > insertText > label
 ---
@@ -200,11 +207,7 @@ local function get_completion_word(item, prefix, match)
           item.filterText and vim.fn.match(item.label, '^\\k') == -1 and item.filterText
           or item.label
         )
-      if item.filterText and not match(word, prefix) then
-        return item.filterText
-      else
-        return word
-      end
+      return fallback_filtertext(item, word, prefix, match)
     else
       return item.label
     end
@@ -212,15 +215,9 @@ local function get_completion_word(item, prefix, match)
     local word = item.textEdit.newText
     word = string.gsub(word, '\r\n?', '\n')
     word = word:match('([^\n]*)') or word
-    if item.filterText and not match(word, prefix) then
-      return item.filterText
-    end
-    return word
+    return fallback_filtertext(item, word, prefix, match)
   elseif item.insertText and item.insertText ~= '' then
-    if item.filterText and not match(item.insertText, prefix) then
-      return item.filterText
-    end
-    return item.insertText
+    return fallback_filtertext(item, item.insertText, prefix, match)
   end
   return item.label
 end

test/functional/plugin/lsp/completion_spec.lua

@@ -457,10 +457,14 @@ describe('vim.lsp.completion: item conversion', function()
 
   it('works on non word prefix', function()
     local completion_list = {
-      { label = ' foo', insertText = '->foo' },
+      { label = ' foo', insertText = '->foo', sortText = '1' },
+      { label = ' bar', insertText = '->bar', filterText = 'bar', sortText = '2' },
     }
     local result = complete('wp.|', completion_list, 0, 2)
-    eq({ { abbr = ' foo', word = '->foo' } }, extract_word_abbr(result.items))
+    eq({
+      { abbr = ' foo', word = '->foo' },
+      { abbr = ' bar', word = '->bar' },
+    }, extract_word_abbr(result.items))
   end)
 
   it('trims trailing newline or tab from textEdit', function()