vim-patch:8.1.0822: peeking and flushing output slows down execution (#25629)

Problem:    Peeking and flushing output slows down execution.
Solution:   Do not update the mode message when global_busy is set.  Do not
            flush when only peeking for a character. (Ken Takata)

cb574f4154

test/functional/lua/ui_event_spec.lua

@@ -77,8 +77,7 @@ describe('vim.ui_attach', function()
     }
 
     feed '<c-y>'
-    -- There is an intermediate state where the 'showmode' message disappears.
-    screen:expect_unchanged(true)
+    screen:expect_unchanged()
     expect_events {
        { "popupmenu_hide" };
     }