UI Text picking

[bytemunch]

Objective

Add text picking backends.

Fixes #17477 + fixes #17706

Solution

Write a backend that emits text specific pointer events for UI text.

Testing

examples/picking/text_picking.rs

Next steps

• More visual example, perhaps showing selectable text showcasing dragging observers.

• Performance concerns: Iterating every glyph on every hover event is a LOT of iterating. Maybe leave getting a PositionedGlyph from a Cursor up to a helper function

• Fix window scale factor issues

• Consider fully decoupling from Pointer, Like read PointerHits directly, or even create a separate TextPointerHits channel. Lots of duplicated functionality though.

---

Showcase

Currently returns a cosmic Cursor in a new TextPointer event that can be used in observers similar to Pointer events.

[github-actions]

The generated examples/README.md is out of sync with the example metadata in Cargo.toml or the example readme template. Please run cargo run -p build-templated-pages -- update examples to update it, and commit the file change.

[alice-i-cecile]

I'm merging #17681; can you update this PR to use that? :)

[bytemunch]

Yeah absolutely, thanks :)

[alice-i-cecile]

And I'm merging #17748, which will create some more merge conflicts for you <3

[NicTanghe]

interesting.
I wonder if i can make this output the position starting from the index of the char in a text.

[bytemunch]

Yeah it's possible with some APIs in this PR, this is very work-in-progress at the moment so I wouldn't rely on anything here being available in bevy any time soon if at all, but everything I've implemented should be possible in third party space. Getting the PositionedGlyph from a text pick is and will be possible, just working out the best way of giving access to that. Once you have access to the glyph it's just a bit of transform math and you should have the location :)

[bytemunch]

I think I'll refocus this PR to just UI text picking, once all the kinks are done there I'll have another look at Text2d, code changes getting a bit heavy