Please throw a warning if iron-list isn't explicitly sized

[ernsheong]

My iron-list was happily rendering every single item in the list all this time until I hit extremely slow UI (1 fps) on Firefox 57, with a large number of items (but only 180 items?) in the list (also perhaps due to polyfill). Spent hours on the bug wondering why my iron-list was rendering everything and not doing what it was supposed to do.

I would like to suggest throwing a warning or error if the iron-list has not been explicitly sized to detect such failures early, especially when inadvertent changes to CSS are made that can cause iron-list to regress.

[keanulee]

Not sure how to reliably use JS to determine if an element is explicitly sized at runtime. Also, you should check if the scroller is explicitly sized, since it's not always the case that iron-list itself is explicitly sized (e.g. it can delegate scrolling to the document).

[ernsheong]

Hmm I think maybe a check that iron-list is rendering everything given to it suffices (length of backing array === no. of virtual row children), when length of item list exceeds a threshold (say 30?) It's just a warning so we need not worry too much about false positives.