Layout, measurement, and IME lanes

Purpose

Compile the evidence that layout and IME formatting are their own lanes, not generic overlay leftovers.

Strongest evidence

• Premirror splits snapshot, measure, compose, and viewport rendering.
• Pretext splits one-time text preparation from hot-path layout arithmetic.
• EditContext makes text updates, selection updates, layout updates, and text-format feedback explicit.

What this means

1. Layout stays derived

Premirror and Pretext both say the same thing:

• layout is computed from document state
• layout should not become the editor model itself

That argues against mixing pagination/layout concerns into the overlay ownership model.

2. Measurement is not “just another decoration”

Pretext proves measurement wants its own precompute/hot-path split.

That matters for huge-doc and page-layout lanes later, even if it is not the current blocker.

3. IME formatting is a separate platform lane

EditContext makes explicit something web editors often blur:

• text input services can request formatting feedback
• geometry and selection synchronization are explicit responsibilities

That is useful because it shows browser/IME formatting pressure is not well modeled as ordinary app decorations.

Take for Slate v2

• keep layout derived
• keep IME/platform formatting pressure distinct from normal overlay API design
• do not let future pagination work distort the current overlay ownership model