patch 9.1.0936: cannot highlight completed text

Problem:  cannot highlight completed text
Solution: (optionally) highlight auto-completed text using the
          ComplMatchIns highlight group (glepnir)

closes: #16173

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_popup.vim b/src/testdir/test_popup.vim
index 69228e1..bd36957 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -1712,4 +1712,49 @@
   call StopVimInTerminal(buf)
 endfunc
 
+func Test_pum_matchins_higlight()
+  CheckScreendump
+  let lines =<< trim END
+    func Omni_test(findstart, base)
+      if a:findstart
+        return col(".")
+      endif
+      return [#{word: "foo"}, #{word: "bar"}, #{word: "你好"}]
+    endfunc
+    set omnifunc=Omni_test
+    hi ComplMatchIns ctermfg=red
+  END
+  call writefile(lines, 'Xscript', 'D')
+  let buf = RunVimInTerminal('-S Xscript', {})
+
+  call TermWait(buf)
+  call term_sendkeys(buf, "S\<C-X>\<C-O>")
+  call VerifyScreenDump(buf, 'Test_pum_matchins_01', {})
+  call term_sendkeys(buf, "\<C-E>\<Esc>")
+
+  call TermWait(buf)
+  call term_sendkeys(buf, "S\<C-X>\<C-O>\<C-N>")
+  call VerifyScreenDump(buf, 'Test_pum_matchins_02', {})
+  call term_sendkeys(buf, "\<C-E>\<Esc>")
+
+  call TermWait(buf)
+  call term_sendkeys(buf, "S\<C-X>\<C-O>\<C-N>\<C-N>")
+  call VerifyScreenDump(buf, 'Test_pum_matchins_03', {})
+  call term_sendkeys(buf, "\<C-E>\<Esc>")
+
+  " restore after accept
+  call TermWait(buf)
+  call term_sendkeys(buf, "S\<C-X>\<C-O>\<C-Y>")
+  call VerifyScreenDump(buf, 'Test_pum_matchins_04', {})
+  call term_sendkeys(buf, "\<C-E>\<Esc>")
+
+  " restore after cancel completion
+  call TermWait(buf)
+  call term_sendkeys(buf, "S\<C-X>\<C-O>\<Space>")
+  call VerifyScreenDump(buf, 'Test_pum_matchins_05', {})
+  call term_sendkeys(buf, "\<C-E>\<Esc>")
+
+  call StopVimInTerminal(buf)
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab