perf: gallery virtualization

[isekovanic]

🎯 Goal

Our image gallery was never properly virtualized and mounted one slide component per asset for all loaded media (assets.map(...)), so scroll/swipe jank scaled with the asset count. On a media heavy channel roughly 40% of frames were janky, driven by O(N) animated style worklets rerunning on the UI thread every frame. This bounds that cost so gallery performance no longer degrades as channels accumulate media. This was relatively fine before, when ChannelDetails hadn't been introduced yet but now poses an actual challenge.

🛠 Implementation details

• Extracted the slide list into a windowed GalleryPager that mounts only the slides within PAGER_WINDOW_RADIUS of the current index, instead of all N. Mounted slides drop from ~N to ~9.
• A single leading spacer reproduces the flex width of the skipped slides, so the rendered slides keep their exact natural positions. The per slide transforms (useAnimatedGalleryStyle) are a pure function of index + flex position, so windowing the mount should be fine
• GalleryPager subscribes to currentIndex itself, so paging rerenders only the small slide list, never the parent (gesture objects/GestureDetector stay stable).
• The existing per slide shouldRender load gate is retained (image +-3, video +-1) for now so windowing bounds the mount; shouldRender still bounds content load (notably capping live native video players within the mounted set).

Measured on a debug SampleApp (on a media heavy channel), provided below are the results (naturally, taken from the best 5 runs against baseline and the worst 5 runs against this branch across of many, many runs):

Metric	Before	After	Improvement
Janky frames	40.4%	14.1%	−65% (−26 pts)
Median frame time	42 ms	21 ms	2× faster (−50%)
90th-pct frame time	79 ms	46 ms	−42%
95th-pct frame time	95 ms	67 ms	−29%
99th-pct frame time	150 ms	133 ms	−11%
Missed vsyncs	154	57	2.7× fewer (−63%)
Frames rendered (same path)	856	1,039	+21% throughput
Mounted slide components	~165	~9	~18× fewer

Jank no longer scales with asset count (after jank is flat across N, where before it rose).

🎨 UI Changes

iOS

Before	After

Android

Before	After

🧪 Testing

☑️ Checklist

• I have signed the Stream CLA (required)
• PR targets the develop branch
• Documentation is updated
• New code is tested in main example apps, including all possible scenarios
  • SampleApp iOS and Android
  • Expo iOS and Android

[oliverlaz]

🔥 🔥 🔥

[Stream-SDK-Bot]

SDK Size

title	develop	branch	diff	status
js_bundle_size	1959 KB	1960 KB	+759 B	🔴