feat(feed): content-visibility experiment for long feeds

[rebelchris]

What

Experiment (feed_content_visibility, GrowthBook, default false) that adds content-visibility: auto + contain-intrinsic-size to feed cards so the browser skips layout/paint for off-screen cards on long vertical feeds.

Why

Reports of long feeds becoming slow/unresponsive after scrolling through many posts (ENG-1676, ENG-1681). At ~1000 posts the entire flattened list is in the DOM with no virtualization, so every scroll frame pays layout+paint across all cards. This bounds the render cost at minimal risk without rewriting the col-span grid or scroll logic.

Notes

• Control is unchanged: when the flag is off (or feed is horizontal), non-widened items render bare as today — the wrapper only appears in the variant.
• Grid cards keep the h-full equal-height-row pass-through; list cards stack at natural height.
• Bounds render cost, not memory — DOM nodes still accumulate. If iOS "refreshes to top" turns out to be OOM rather than CPU jank, escalate to row virtualization.

🤖 Generated with Claude Code

Preview domain

https://feat-content-visibility-feed.preview.app.daily.dev

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Project	Deployment	Actions	Updated (UTC)
daily-webapp	Error		Jun 19, 2026 1:37pm

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Project	Deployment	Actions	Updated (UTC)
storybook	Ignored		Jun 19, 2026 1:37pm