# Objective
The default for `ContentSize` should have the `measure_func` field set
to `None`, instead of a fixed size of zero. This means that until a
measure func is set the size of the UI node will be determined by its
`Style` constraints. This is preferable as it allows users to specify
the space the Node should take up in the layout while waiting for
content to load.
## Solution
Derive `Default` for `ContentSize`.
The PR also adds a `fixed_size` helper function to make it a bit easier
to access the old behaviour.
## Changelog
* Derived `Default` for `ContentSize`
* Added a `fixed_size` helper function to `ContentSize` that creates a
new `ContentSize` with a `MeasureFunc` that always returns the same
value, regardless of layout constraints.
## Migration Guide
The default for `ContentSize` now sets its `measure_func` to `None`,
instead of a fixed size measure that returns `Vec2::ZERO`.
The helper function `fixed_size` can be called with
`ContentSize::fixed_size(Vec2::ZERO)` to get the previous behaviour.
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