Finding when Rendering done is not consistent with Qt version

When trying to find out when rendering is complete using a small Java script which is injected to know if rendering is fully done, but this seems like it behaves different with different version of Qt and different on windows and mac.

Attached is the example which can be used to reproduce this. When you start typing in LineEdit it webengine will render some Symbols and once rendering is done it calls grab() on webview and displays in the application.

If you run this example on windows using Qt5.15.15 on windows it works without issue. But same when run with Qt6.7 this does not work. Same on Mac.

If we use one time Timer to delay like 2 seconds it works.