RFC: separate the actual items and meta-items at the API level

[kvark]

Currently, every display item looks like this:

pub struct DisplayItem {
    pub item: SpecificDisplayItem,
    pub clip_and_scroll: ClipAndScrollInfo,
    pub info: PrimitiveInfo<LayoutPixel>,
}

pub struct ClipAndScrollInfo {
    pub scroll_node_id: ClipId,
    pub clip_node_id: Option<ClipId>,
}

pub struct PrimitiveInfo<T> {
    pub rect: TypedRect<f32, T>,
    pub clip_rect: TypedRect<f32, T>,
    pub is_backface_visible: bool,
    pub tag: Option<ItemTag>,
}

The SpecificDisplayItem is a giant enum of all possible things we may need, including stuff that only describes the metadata, or context for other items:

• the reference & sticky frames don't need any clip information, like clip_rect or clip_node_id
• the pop* variants don't need any accompanying information at all
• setting gradient stops don't need any "standard" primitive info
• etc

---

This RFC is for making a top-level enum like this:

enum DisplayItem {
  // stacking context and shadows
  Push/PopStackingContext,
  Push/PopShadow,
  // reference frames, sticky and scroll frames
  DefineReference/Sticky/ScrollFrame
  // clips, clip chains, and gradient stops
  DefineClip/ClipChain,
  SetGradientStops,
  Primitive { 
    primitive: SpecificPrimitive,
    clip_and_scroll: ClipAndScrollInfo,
    info: PrimitiveInfo<LayoutPixel>,
  }
}

enum SpecificPrimitive {
  Rectangle,
  Image,
  Text,
  Line,
  /// etc
}

The result I expect from these changes would be:

• cleaner API that matches the internal logic. The clients don't need to guess if a clip_node_id is used for a particular item, ignored, or asserted upon, for example.
• reduced overhead for serializing, sending over, and deserializing DL data.

[Gankra]

I expect you'll quickly get annoyed by DisplayItem::Primitive not being a real type and extract it out to a struct but otherwise yeah this seems like a nice cleanup.

I'm not sure if it's correct for Shadows to be treated as metadata? They have an offset so are clipped slightly differently than their contents. But I've lost track of how we handle this.

[gw3583]

There's a few details like @gankro mentioned above, but I like the idea in general.