Window.ShowDialog sometimes blocks several seconds when there is a task frequently update UI

* .NET Core Version: Preview5
* Windows version: 版本6.1 (内部版本 7601：Service Pack 1)
* Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

**Problem description:**
I have a Window to display high frequency market data.
I use Dispatcher.InvokeAsync(... , DispatcherPriority.Background) to update the UI.

Sometimes if I click some button to show a dialog window, it will block the whole application with serveral seconds, and after the dialog has shown, everything goes OK.

Others such as ContextMenu, ComboBox works fine.
**Actual behavior:**

**Expected behavior:**

**Minimal repro:**