The https://www.qt.io/qt-demos page embeds the Slate application. When scrolling down to see the Slate app, the app may end up without a visible canvas.

From sorvig:

The white canvas zooms on scroll events, and it can happen that scrolling down on the page makes it invisible, depending on the event flow. I don't think its specific to Safari.

There are a couple of ways to fix it, like filtering the events or disabling wheel event canvas zoom for this build of the example.

Another alternative is to modify the Slate app to always keep the canvas visible during zoom events.