Rendering and interaction rules

Each of these has cost someone a bug.

1. Never autofocus. The composer owns the caret. An inline block that grabs focus eats the sentence the human was typing.
2. Height must not change on stamp. Reserve the space. A block shrinking three screens up yanks the scroll position out from under the reader.
3. No layout animation. Opacity and colour only; drop even those under prefers-reduced-motion.
4. Reserve image space before load. A gallery that pops in at natural height reflows the whole page.
5. Icons are vectors, never glyphs. A text icon falls out of the host’s font stack into a fallback and draws a tofu box.
6. Number keys 1-9 select when focused; Enter submits; arrows drive @scale; Escape does nothing — there is nothing to dismiss, and people press it reflexively.
7. Real <button> elements. select=one is a role="radiogroup", select=many a <fieldset> of checkboxes.
8. Stamped leaves the tab order but stays readable to a screen reader.
9. Full width of the message, not the viewport. A block is part of a message.
10. Mobile: 44px hit targets, hints wrap rather than truncate, galleries go two-up, @upload offers the camera.
11. Drag is never the only path.
12. Single-select never gets a submit button. The click is the answer; a button would be a second, meaningless step.