Restore content offset after background snapshot on iOS #55026
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In iOS 26, backgrounding an app triggers an app snapshot [1], which will be taken in all supported device orientations. When in landscape mode, the OS will make a layout pass in portrait followed by another in landscape (and vice versa), taking a snapshot of each. If scrolled to the bottom of a ScrollView in landscape mode, the content offset will be incorrect on resume, as UIScrollView updates the offset during the portrait layout pass to prevent overscroll. When the next layout pass is made, the offset is not reset to the original value. Store the content offset on update when foregrounded. When backgrounded, override the content offset with the stored content offset, clamped to the content size. When the second snapshot is taken (in the original orientation), the original content offset is restored. [1] https://developer.apple.com/documentation/uikit/preparing-your-ui-to-run-in-the-background#Prepare-your-UI-for-the-app-snapshot
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Summary:
In iOS 26, backgrounding an app triggers an app snapshot [1], which will be taken in all supported device orientations. When in landscape mode, the OS will make a layout pass in portrait followed by another in landscape (and vice versa), taking a snapshot of each.
If scrolled to the bottom of a ScrollView in landscape mode, the content offset will be incorrect on resume, as UIScrollView updates the offset during the portrait layout pass to prevent overscroll. When the next layout pass is made, the offset is not reset to the original value.
Store the content offset on update when foregrounded. When backgrounded, override the content offset with the stored content offset, clamped to the content size. When the second snapshot is taken (in the original orientation), the original content offset is restored.
Fixes #54979.
[1] https://developer.apple.com/documentation/uikit/preparing-your-ui-to-run-in-the-background#Prepare-your-UI-for-the-app-snapshot
Changelog:
[IOS] [FIXED] - Scroll position not maintained after resume on iOS
Test Plan:
Reproducer: https://snack.expo.dev/@mhoran/restless-green-graham-crackers
Per #54979:
Without this patch, the ScrollView position will change, due to the background snapshot taken in portrait mode.
Screen recording: https://drive.google.com/file/d/1WrEXNSF2dcYswtKrlM6YJglGiOzoGq8u/view?usp=drive_link.