I noticed that a simple collapsed dock I have is still showing a bit of the contents when the page loads. If I expand it and collapse it, then it will display properly. Version I have is 2019.2.514
I checked and noticed it was using the Lightweight render mode and that it would be missing "overflow:hidden" in the css in the initial page load.
Attached are images showing the generated CSS at the page load and after expanding and collapsing again.