In many cases, fullscreen interactions with WebEngine (widgets) are poor when the <video> tag is in use. Poor behavior consists of failure to resize the page when fullscreen results in a change of client area, and intermittently, frozen video playback after window resize. See attached sample.
5.7.0 behaves slightly better than 5.6.0, but is still not correct.
Creation of a QStatusBar object (it does not need to be visible or parented to anything, one simply needs to exist) improves behavior, especially the frozen video issue.