Windows: When a QWebView is already visible then loading a page that requires a plugin doesn't always show the plugin straight away

When a QWebView is already visible then loading a page that requires a plugin doesn't always show the plugin straight away. This seems to be somewhat inconsistent but with the attached example it reproduces every time.

You may need the patch from https://codereview.qt-project.org/#change,74890 in order to ensure it is shown correctly when it does show up (otherwise it may be 0x0).