DevTools assumes the framerate for a device does not change once the performance page opens

Some devices like a Pixel4 can intentionally switch between 60 and 90fps.

We currently appear to only query for the refresh rate on startup. We probably need to receive the target refresh rate with each frame timing even or receive separate events when the target frame rate changes.
https://github.com/flutter/devtools/blob/64aefd56202560727758bac7e5629841c902e5fa/packages/devtools_app/lib/src/performance/performance_controller.dart#L200

Fyi @kenzieschmoll, @jonahwilliams

Start at packages/devtools_app/lib/src/performance/performance_controller.dart around the linked line and trace how the performance page obtains the device refresh rate at startup. Investigate the frame-timing flow and whether it exposes target refresh-rate changes. Done means the page reflects a device switching between 60 and 90 fps after it opens.

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