dom-repeat initial-count throttling fails when pushing new items

[kevinpschaaf]

Description

When adding new items to an already-rendered <dom-repeat> with initial-count, chunking is re-initialized. This has two effects: First, any already rendered items past the initial count are removed, then because the already-rendered items "render" in a very short time, the average time per item is very low, causing the remainder of the list to be rendered in one shot, thus chunking is defeated for newly added items.

Live Demo

https://glitch.com/edit/#!/kaput-hollyhock-o6tozsgx4a?path=index.html:9:0

Expected Results

• After adding more items (using either push or spread method), the new items are chunked in.
• The global id of the initially rendered items do not change (meaning they haven't been removed & re-created)

Actual Results

• The added items are rendered in one shot.
• The global id of the initially rendered items change (meaning they were removed & re-created)

Versions

• Polymer: v3.x