I see this black line which takes me down to the metric an shoes that page is interactive. what metric this is related to. Here is the test result I am sharing the URL https://www.webpagetest.org/result/190930_DP_5ff1de80bcae50e5ea6589844ac60a5d/1/details/
This is defined as the ability of the user to interact with the page once it becomes visible.
In WPT there are 2 color states to this metric - the red means that the user can see the page but if they tried to interact with it either to scroll or click/tap, nothing would happen. As you can imagine that would be frustrating to the user.
The green state shows areas where the user is able to interact with the page.
Here is a more technical explaination:
Also an explanation as to why it happens through Long Tasks