Use scrollLeft to detect when scroll indicators should appear

[ahmacleod]

If the table element has a negative margin-left style, the left scroll indicator will always show, even when scrolled all the way to the left. This negative margin trick is commonly used in conjunction with border-spacing and an overflow element in order to eliminate leading whitespace in the leftmost column of a table (border-spacing cannot be overridden at a cell level).

By examining scrollLeft instead of the bounding box positions of the table & overflow elements, the same effect is achieved without making any assumptions about the relative positions of elements.

[kpfefferle]

I tested this out in my consuming app, and it seems to work well. This seems like a more robust solution for the long-term since it accounts for this additional use case 👍🏻

[ahmacleod]

Made redundant by #854