The tile placeholder method didn't work well because the component doesn't know how big the page size is, so we always end up rendering 1 placeholder tile, followed by N (= page size) placeholders (if not yet resolved). It's pretty ugly.
Removed comments:
- Incorrect/misleading: "injected item" does not mean "ads". It can be any React component that you want to put at a specific index.
- Redundant: don't explain the syntax. It's very annoying having to maintain the comment when the logic changes.
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