HTML Select controls open slowly / draws in wrong place

ORIGINAL REPORT:
I'm building an application that makes extensive use of user input, including many select controls. In QWebView, select controls were very responsive. Under WebEngine, many things are vastly improved over QWebView, but opening select controls is strangely slow.

I found an example online that demonstrates the issue:
http://www.syntaxsuccess.com/angular-2-samples/#/demo/form

Additionally, in one of my QWebEngine views, I find that if I scroll down the page before clicking the select, the pop-up menu is not positioned correctly when it appears – even showing up outside of Qt's window (!!). Screenshot attached. Scroll down the image and notice the menu – floating in the middle of nowhere – showing "Dan Berman" and "John Smith, MD". This is actually a result of clicking on the "Finalizing Study" pop-up menu much farther up the image.

This doesn't happen when the exact same web page is loaded into the QWebEngine view on the panel to the left of the same application. (I have 2 separate views.)

UPDATE:
See comments section for instructions on how to easily modify Qt's demobrowser example to demonstrate this issue in Qt 5.6.0 final. (Just add a QSplitter, and put the WebEngine view in the right side. Drag splitter left or right... HTML select will draw in wrong spot.)