patch 8.2.5064: no test for what 8.1.0052 fixes

Problem:    No test for what 8.1.0052 fixes.
Solution:   Add a test. (closes #10531)

src/testdir/test_mapping.vim

@@ -1685,4 +1685,34 @@ func Test_expr_map_escape_special()
   nunmap …
 endfunc
 
+" Testing for mapping after an <Nop> mapping is triggered on timeout.
+" Test for what patch 8.1.0052 fixes.
+func Test_map_after_timed_out_nop()
+  CheckRunVimInTerminal
+
+  let lines =<< trim END
+    set timeout timeoutlen=400
+    inoremap ab TEST
+    inoremap a <Nop>
+  END
+  call writefile(lines, 'Xtest_map_after_timed_out_nop')
+  let buf = RunVimInTerminal('-S Xtest_map_after_timed_out_nop', #{rows: 6})
+
+  " Enter Insert mode
+  call term_sendkeys(buf, 'i')
+  " Wait for the "a" mapping to timeout
+  call term_sendkeys(buf, 'a')
+  call term_wait(buf, 500)
+  " Send "a" and wait for a period shorter than 'timeoutlen'
+  call term_sendkeys(buf, 'a')
+  call term_wait(buf, 100)
+  " Send "b", should trigger the "ab" mapping
+  call term_sendkeys(buf, 'b')
+  call WaitForAssert({-> assert_equal("TEST", term_getline(buf, 1))})
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete('Xtest_map_after_timed_out_nop')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab