I reckon it’s a mistake. Hiding scrollbars dynamically is unpleasant, and the degenerate cases like `position: fixed` seen on this particular page make it very unpleasant.
Embrace what the web exposes. Allowing document scrolling underneath is distinctly the lesser of two evils. With a bit of effort, you can block scrolling underneath in almost all cases, and I reckon that’s good enough and much better than what there is currently. (Also try asking browser makers, “y’know this ‘top layer’ thing you made for <dialog> modal presentation and fullscreen? Wouldn’t it be nice if we could prevent scrolling underneath a ::backdrop?”)
Embrace what the web exposes. Allowing document scrolling underneath is distinctly the lesser of two evils. With a bit of effort, you can block scrolling underneath in almost all cases, and I reckon that’s good enough and much better than what there is currently. (Also try asking browser makers, “y’know this ‘top layer’ thing you made for <dialog> modal presentation and fullscreen? Wouldn’t it be nice if we could prevent scrolling underneath a ::backdrop?”)