Refresh issue in WebEngineView when window.open() is called

This is a regression from Qt 5.13.

When opening a new window from javascript with window.open(), the newly opened window shows nothing, though the content is actually loaded. It looks like a refresh issue.

To reproduce with quicknanobrowser:

1. start quicknanobrowser
2. open DevTool pane
3. in the console, type window.open("https://www.google.com")

The result is that a new tab opens, but stays blank. However, switching back-and-forth between tabs is enough to trigger a visual redraw and see the content.