Problems with the text system, change detection and `MeasureFunc`

[ickshonpe]

Bevy version

main d7d983

Details

UI layout changes don't get propagated correctly. Sometimes nodes completely disappear.

Worst offender, but definitely not the only bug, is this line in text_system:

Query<Entity, Or<(Changed<Text>, Changed<Style>)>>,

A lot of changes to the Style properties won't result in changes to the geometry of the node, and the text gets recomputed needlessly. On the other hand, non-local changes that affect the geometry of the text node aren't recognised and the text doesn't get recomputed when it should be.

An example is that if you set a text node to fill 50% of its parent node, and you change the size of the parent node, the text layout won't be recalculated.

[ickshonpe]

There are other bugs in the UI that cover up this bug partially, which makes it quite hard to create a minimal example.

I've fixed some of the text problems properly in a fork, like the issue where only pixel values and not percentages can be used to constrain text nodes. But then when you've fixed that, this recomputation bug becomes much more apparent, and a lot of everything else breaks.