Source: Indenting fix
Enhancement: Better scrollbar handling

Avoid having excess space just in case there's a particularly wide
scrollbar.

Add the relevant width to pages without a scrollbar, rather than
subtracting it from pages with one. That avoids needing to have an
explicit 'margin of error' added in the first place.

This reverts parts of 0f81f3d and 34201f8.
Source: "Scroll bar" as 2 words

As per 'OED'.