Add support for grid-gap, grid-row-gap and grid-column-gap

[RamIdeas]

This PR adds support for the gap properties mentioned 🎉

This only took under an hour to implement and a bit longer to (manually) test a little thanks to your comment

If you could offer similar help for any other issues mentioned in #50 I would be glad to have another go!

[FremyCompany]

Awesome! Let me take a look

[FremyCompany]

By the way, if you have some demo pages you were trying your gap implementation on, feel free to add it/them to the demo/css-grid folder!

[magicznyleszek]

@FremyCompany @RamIdeas any ETA on this, any problems to be solved?

[FremyCompany]

See the PR comments, I think some of them are still pending

[FremyCompany]

https://github.com/FremyCompany/css-grid-polyfill/pull/51#discussion_r139486370

[RamIdeas]

First off, apologies for letting this linger for so long. I had trouble finding spare time to pick this up again.

I think I've successfully accounted for *-gap percentage values now by introducing fullDistributableSize -- is what I've done what you had in mind?

I also noticed a slight issue in regards to grid-column-gap being specified when the grid container does not have an explicit height (what the spec calls the "logical height" having a value of auto). The first version of the spec mentions the expected behaviour under "length-percentage" here. I've forked Rachel Andrews' pen with an explanation here.

[FremyCompany]

Thanks! There is still one more thing to check about the negative sizes, and then I think we can merge this.

Percentage resolution with auto height is probably still broken, but making this work doesn't seem very critical to me to be able to merge this. If someone ends up using this and finds the issue, it can be corrected then.

[RamIdeas]

Hey have you had a chance to review the changes? This should be ready to merge 😄

[FremyCompany]

I'm looking at this sometime this weekend :)

[FremyCompany]

Thanks! 💚