This issue pre-dates MDL-73771, but is made more obvious with the large gap that now exists between the "<" block button, and the very narrow page scrollbar:

Firefox (Ubuntu)

Chrome (Ubuntu)

It's because of this line:

    // Firefox.

    scrollbar-width: thin;

For reference, see this from MDN: https://developer.mozilla.org/en-US/docs/Web/CSS/scrollbar-width

Accessibility concerns

Use this property with caution — setting scrollbar-width to thin or none can make content hard or impossible to scroll if the author does not provide an alternative scrolling mechanism. While swiping gestures or mouse wheels can enable scrolling on such content, some devices have no scroll alternative.

This is the current issue, it's a cause of great frustration trying to use a mouse to click/drag an extremely narrow page element