Support Overflow::Scroll

[nicoburns]

Objective

• To make it easier to use Taffy for UI trees containing scrollviews on systems that have scrollbars that take up space in the layout.
• To support layouts containing overflow: scroll for those wanting to faithfully render web layouts.

Tasks

• Add style types
• Implement for leaf algorithm
• Implement for flexbox algorithm
• Implement for grid algorithm

Context

• Part 2 of Support the overflow property #304.
• Follows on from Support Overflow::Hidden #424

Feedback wanted

• scrollbar_width deviates from the spec. The spec only allow auto, thin or none and the user agent must decide what actual widths these correspond to. Whereas I've allowed a pixel value to be specified. I think this difference is justified because Taffy is aimed at "user agents" rather than end users.
• I've made scrollbar_width a u8 to save on space. Does this seem reasonable, or do you think I should make it an f32 like everything else? scrollbar_width has been converted to f32 in response to feedback.
• I've used a single scrollbar_width for both axis (the overflow property still applies separately in each axis). So IF there is a scrollbar in both axis then they must both be the same size, does this seem reasonable.

[alice-i-cecile]

scrollbar_width deviates from the spec. The spec only allow auto, thin or none and the user agent must decide what actual widths these correspond to. Whereas I've allowed a pixel value to be specified. I think this difference is justified because Taffy is aimed at "user agents" rather than end users.

I agree: that's an unhelpful spec setup.

I've made scrollbar_width a u8 to save on space. Does this seem reasonable, or do you think I should make it an f32 like everything else?

I think we should probably be consistent rather than clever here. I can't imagine this making a performance difference, and fractional sizes are reasonable.

I've used a single scrollbar_width for both axis (the overflow property still applies separately in each axis). So IF there is a scrollbar in both axis then they must both be the same size, does this seem reasonable.

That seems sensible: uneven scroll bars definitely seem wrong.

[alice-i-cecile]

Good docs, clean code. Great tests as always.