QtWebEngine always shows scrollbars on Mac despite system setting

Can be seen with the Quick Nano Browser example on macOS. Build and run, and show a page that is long enough to scroll.

On a Mac, with "Show scroll bars" set to "Automatically", the scrollbar shouldn't be visible unless you initiate scrolling with mouse or trackpad.

However, the scrollbar is always visible. This is not the case in other Chromium-based surfaces such as Electron (e.g. Slack) or Chrome. This is a significant problem in our actual QtWebEngine-based product, which has numerous scrolling areas and looks terrible with all these scrollbars (screenshot also attached).