fix(ui-ext): correct message kind in history before vim.ui_attach()

src/nvim/message.c

@@ -3151,6 +3151,7 @@ int msg_end(void)
 void msg_ext_ui_flush(void)
 {
   if (!ui_has(kUIMessages)) {
+    msg_ext_kind = NULL;
     return;
   }
 

test/functional/lua/ui_event_spec.lua

@@ -117,4 +117,31 @@ describe('vim.ui_attach', function()
     })
     eq(0, helpers.eval('v:shell_error'))
   end)
+
+  it('can receive accurate message kinds even if they are history', function()
+    exec_lua([[
+    vim.cmd.echomsg("'message1'")
+    print('message2')
+    vim.ui_attach(ns, { ext_messages = true }, on_event)
+    vim.cmd.echomsg("'message3'")
+    ]])
+    feed(':messages<cr>')
+    feed('<cr>')
+
+    local actual = exec_lua([[
+    return vim.tbl_filter(function (event)
+      return event[1] == "msg_history_show"
+    end, events)
+    ]])
+    eq({
+      {
+        'msg_history_show',
+        {
+          { 'echomsg', { { 0, 'message1' } } },
+          { '', { { 0, 'message2' } } },
+          { 'echomsg', { { 0, 'message3' } } },
+        },
+      },
+    }, actual, inspect(actual))
+  end)
 end)