QWebEnginePage::Back/Forward button doesn't become enabled when <frame>/<iframe> is updated

QWebEnginePage::Back/Forward button doesn't become enabled when <frame>/<iframe> is updated.

How to reproduce :
1. Run the attached sample code (QTBUG81512.zip) with the parameter “test\frame\index.html”
2. Click one of the links on the left side. e.g. "right2" -> In the right view, the frame is switched to right2.

Expected behavior:
Back button in the menu bar becomes green and enabled.

Actual behavior
The button stays gray and disabled.

The button works when

• Click the link "Qt" on the left side. This enables to trace all the action history up to the beginning. That is, the right side changes Qt -> right2 -> right1
• Click right mouse on the left side, This shows a menu where the back button is enabled. ( See webEngineHistory.png )

The same thing can be tested for <iframe> with the parameter "test\iframe\index.html"