Fix Session Replay masking during view transitions

[kahest]

Written by @philprime:

Current state of investigation:

Based on given replays visible in Sentry product I could pinpoint the partial frames without masking to screenshots taken during transitions, e.g. during a push-navigation of a detail view controller.

By enabling the "Slow Animations" in the debug menu of the iPhone simulator, and increasing the frame-rate of Session Replay from 1 FPS to 10 FPS, I am able to reliably reproduce the issue.

The currently released SDK does not allow configuration of the FPS, and while analyzing the issue at hand, I also had to fix the SDK to support higher frame-rates due to calculation and timing errors. I'll create a separate PR for that.

My current assumption is that during the transitions the view hierarchy is replaced by a snapshot view, which uses the same visual representation, but does not use an actual view hierarchy we can use to calculate redaction areas. Please see the discussion of UIView.snapshotView(afterScreenUpdates:) for details on snapshot views. This assumption is not yet proven, but the current main lead.

I pushed all the work-in-progress changes to philprime/sr-masking-analysis and for easier viewing opened up #5073

Next steps:

• Analyze if unmasked view is a snapshot view + how to mask it (maybe fully mask during transition)
• Disable screenshots taken during transitions (Potential Lead: https://stackoverflow.com/a/5526556/3515302)
• Analyze potential inverse approach in masking to convert from the opt-out to an opt-in approach (i.e. instead of masking areas of the clear screenshot which are sensitive, cut out areas that are safe to show)
• Create separate PR with fixes for higher frame rates

[kahest]

related: #5023

[philprime]

@kahest I updated the description of this issue with the details of the current state.