[ipad] Fix unreachable content on scroll

[kimprice]

Fixes https://github.com/microsoft/pxt-microbit/issues/5706

Upload target: https://makecode.microbit.org/app/85162957a6b556f452ce00deebb0d4425fb22de7-7f0c9fae0c--eval

Note: the behavior is different when using the dev tools to simulate an ipad. To see the true behavior, try the upload target on an actual ipad.

[riknoll]

@kimprice if you have access to a mac, you can also use the xcode ipad simulator to test bugs like this! works well if you don't happen to have an ipad (as i do not)