Example page: crop box "jumps"

This may be a known issue with the [example page](https://github.github.io/image-crop-element/examples/). If it's not worth fixing please feel free to close. At first I thought there was something wrong with the component but it only manifests when the image crop inputs are visible- the updateDimensions logic doesn't account for the height offset they cause.

https://github.com/github/image-crop-element/assets/1306747/316ab9a6-ea81-4552-ae3b-e09c02f90223