Support auto track definitions

[shyndman]

Auto tracks are generated automatically when the placement algorithm places an item outside the range of the templated tracks. This could be useful in some cases.

[erickok]

Which type of track would you pick, though? The relatively heavy IntrinsicContentTrack? Or a FlexibleTrackSizeTrack(1)? Not sure if we can come up with an intuitive default.
Alternatively, in case of 'missing' tracks, we could not crash/error but instead show overflow bars (on debug builds) and otherwise ignore this content?

[shyndman]

Which type of track would you pick, though? The relatively heavy IntrinsicContentTrack? Or a FlexibleTrackSizeTrack(1)? Not sure if we can come up with an intuitive default.

No defaults! We let the user decide.

https://developer.mozilla.org/en-US/docs/Web/CSS/grid-auto-rows

Alternatively, in case of 'missing' tracks, we could not crash/error but instead show overflow bars (on debug builds) and otherwise ignore this content?

Agreed that a crash is a little harsh. Let me think about it.

[nicoburns]

I think this should actually be quite straightforward to implement. All you'd need to do is resize to fit the item your placement grid instead of throwing an error. I have an implementation of this for my CSS Grid implementation here. As you can see, it's quite short and straightforward. And mine is actually quite a bit more complex than I think you'd need as it needs to support negative indexes (adding negative implicit tracks is a massive pain as it means the origin of grid moves!)

For performance reasons I also have some logic to estimate the required size ahead of time (by scanning through the grid placements and spans of every item and finding the min/max - you'd only need the max). The logic for that is here. This doesn't completely guarantee no resizes as auto-placement can still generate more tracks but it helps in the "explicitly placed out of bounds" case.