# Backdrops: living backgrounds for OxygenHub

Status: the engine and the dashboard's Your sky are built (oxygenhub#157,
2026-10-13); the other scenes are proposed (oxygenhub#156).

## 1. What a backdrop is

A backdrop is a quiet, moving picture behind a page's content: a field of
small marks that drift slowly and respond a little to the cursor. It
gives the hub a sense of motion and depth without asking for attention.

Three rules shape everything else:

1. **True, not invented.** The footer says "nothing here is inferred:
   what a page says, the object store says". A backdrop keeps that
   promise. Every mark stands for something real the viewer may see: one
   of their repositories, a change, a push, a runner. Nothing is made-up
   decoration that only looks like data, such as fake chunks or fake
   peers.
2. **Behind, never in the way.** Content is opaque cards on top. A
   backdrop shows only in the margins and the gaps between cards. It never
   takes a click, never moves the layout, and is never needed to read or
   use the page.
3. **Cheap and polite.** It costs well under a millisecond a frame. It
   stops when the tab is hidden, and it is still for people who ask for
   reduced motion. Anyone can turn it off.

## 2. Which pages

| Page | Scene | Shows |
|---|---|---|
| Dashboard | **Your sky** (§5) | Your repositories as clusters; their activity in the last 30 days as the marks around them |
| Signed-out home, Explore | **The hub's sky** | The same scene from public repositories only |
| Sign in, register, device sign-in | **The mark** | The Oxygen mark's two rings with marks orbiting them; brand only, no data |
| A repository's Diffusion tab | **Ripples** | The upstream and its consumers as the Diffusion mark draws them: a pulse goes out on a bump, green or red as its run ends |
| Admin › Runners › Images | **The swarm** | Runners as nodes; chunks moving between them while an image is fetched, from the swarm tracker's live state |
| Not found, server error | **Adrift** | One mark drifting away from the rest |

**Never:** code, file, blame, diff, conflict and proposal pages, the
editor, issues and proposals while being written, settings and admin
tables. Those pages are for reading closely, and motion behind them
costs attention. Repository overviews have none for the same reason;
their README is the picture.

Each scene is its own issue, done one at a time, the dashboard first.

## 3. Colours

A backdrop takes its colours from the page's own tokens, so it is right in
both looks (GitHub, the default, and classic) and in both themes (light,
the default, and dark), and it follows a theme switch at once. Three new
tokens join each look's light and dark sets:

| Token | Use | GitHub light | GitHub dark | Classic (dark) |
|---|---|---|---|---|
| `--bd-mark` | a resting mark | `--mute` at 16% | `--mute` at 14% | `--acc` at 12% |
| `--bd-lit` | a mark near the cursor | `--acc` (#0969da) at 70% | brand cyan #00F0FF at 75% | #00F0FF at 85% |
| `--bd-link` | a line between lit marks | `--acc` at 22% | #00F0FF at 28% | #00F0FF at 35% |

Two more colour rules:

- **The brand gradient (#00F0FF → #0072FF)** is kept for moments: a new
  push arriving, the mark scene. A whole field in it would shout.
- **Status colours are used only where the data has a status:** the
  Diffusion pill's green, amber and red, and a run's outcome. They never
  appear as decoration.

In light mode a backdrop must be barely there: if you notice it before the
content, it is too strong. The contrast test (`tests/contrast.rs`) gains a
check that `--bd-mark` stays under 1.3:1 against `--bg`.

## 4. Motion and the cursor

- **Drift.** Marks move slowly, at most 12 px a second, along gentle
  streams around their cluster. Two streams can cross: their marks pass
  each other without touching, and the change in order means nothing.
  That is how Oxygen's changes behave, which is where this comes from.
- **The cursor lights, it does not pull.** Marks within about 140 px of
  the cursor light up in `--bd-lit`, ramping in over 150 ms and fading
  out over 600 ms. Nothing is attracted or pushed, so the cursor never
  rearranges the picture.
- **Lines mean something.** Among the lit marks, a faint line joins two
  marks only when they belong together in the data: events of the same
  repository that happened one after another and sit within 70 px of
  each other, so a busy repository reads as a constellation, not a
  scribble. Lines fade as the cursor moves on or rests for 2 s.
- **Labels, sparingly.** When the cursor rests near a cluster, its name
  fades in just above the cluster's outer ring in `--mute`, small. This is the only text a backdrop
  draws, and it is never a link; the dashboard's own list is the link.
- **Parallax.** Three depth layers shift by at most 6 px, 3 px and 1 px
  against the cursor's offset from the page's centre. That gives a little
  depth with no feeling of motion sickness.
- **Arrivals.** When the page learns of new activity, its mark appears
  with one soft ring in the brand gradient that widens and fades over
  1.2 s. This is the one moment a backdrop may draw the eye, and only
  briefly.

## 5. The dashboard: Your sky

**Data.** The dashboard handler already loads your repositories and the
activity feed from the database. The backdrop needs a little more of the
same:

- the repositories in Your repositories, by name;
- up to 1,500 events from the last 30 days across them (pushes,
  proposal and issue events, releases, Flow runs), each with its
  repository, kind and time, and for a push the number of changes it
  brought. A push draws one dot for each change, up to 40, and the sky
  draws up to 3,000 marks.

The handler embeds these as JSON in the page (`<script
type="application/json" id="backdrop">`), so the backdrop costs no
second request and never reads a repository's store. That matters:
some stores are slow, such as GCS-backed ones. Private repositories
appear only for viewers who may see them, exactly as the list beside
them does.

**Layout.**

- Each repository is a cluster. The clusters fill the margins beside the
  content column, which the page marks with `data-backdrop-avoid`: in the
  order of their names' hashes, split between the two sides by their
  widths, each side a staggered grid from top to bottom. The same
  repositories in the same window always land in the same places. Where
  a margin is under 180 px, they sit on a ring around the page instead.
- Marks and orbits are drawn at 1.4 times their first size, with orbit
  lanes close together, so each repository is a tight knot (Andy,
  2026-10-13).
- The most active repositories sit in the front layer and the quiet ones
  further back.
- A cluster's events are its marks: today's are brightest and a month
  old are faint.
- Kinds differ by shape, not colour: a change pushed is a dot, a
  proposal event a small diamond, an issue event a ring, a release a
  four-point star, a Flow run a small square, and the repository itself a
  thin ring at the cluster's core.
- A busy repository gets more orbit lanes (6 to 14), so its marks spread
  out instead of piling up.

**Live.** When the page learns of new activity (it already polls for the
notification count), the event joins its cluster with an arrival ring
(§4). Nothing reloads.

## 6. Engine

The engine is Rust compiled to WebAssembly (Andy's choice, 2026-10-13):
`crates/backdrop`, a workspace of its own outside the hub's. `sky.rs` is
the scene with no browser in it, tested natively; `gl.rs` draws it;
`lib.rs` is the browser glue. `make backdrop` builds it for
`wasm32-unknown-unknown` and runs `wasm-bindgen --target web` into
`assets/backdrop/` (about 186 KB of wasm, 60 KB gzipped, and the glue),
which is committed and embedded in the hub, as `make ts` commits the API
types, so neither the hub build nor its offline CI needs the wasm
toolchain. `make backdrop-check` runs its fmt, clippy (wasm32) and tests.

- **Drawing.** WebGL2 with instanced quads for marks (one draw for all,
  each shape a signed distance in the fragment shader) and one line
  buffer for links. Without WebGL2 there is no backdrop, and nothing
  breaks.
- **The canvas.** One fixed full-viewport `<canvas aria-hidden="true">`
  behind `main`, with `pointer-events: none`. Its device pixel ratio is
  capped at 1.5.
- **Frame rate.** It runs at 60 fps while the cursor moves, eases to
  30 fps when the cursor is still, and pauses on `visibilitychange`.
  After 2 minutes without input it draws one last frame and stops until
  the next movement.
- **Reduced motion.** With `prefers-reduced-motion: reduce` it draws one
  still frame with no drift, parallax or arrival rings; lit marks still
  respond, instantly.
- **Budget.** Under 1 ms of script a frame on a 2020 laptop, checked
  with the browser's performance panel before each scene ships.
- **Scenes are data.** A scene is a preset of layout, shapes, counts and
  which data it reads; the engine is shared.

**Why Rust and WASM.** A small script would have done for a few hundred
marks; Rust keeps the whole of Oxygen in one language, lets the scene be
tested like the rest, and is ready for scenes with heavy layout (a
repository's full change graph) where sharing oxy-core's own types would
pay. The cost is kept out of the hub's build by committing the output.

## 7. The viewer's choice

Settings › Appearance gains **Background: Moving (default), Still, Off**,
kept in a `backdrop` cookie read before first paint, like the theme
cookie, so a page never flashes a backdrop it should not have. Still draws
the reduced-motion frame. `Save-Data: on` and a pointer that is coarse
only (a phone) default to Still.

## 8. Order of work

1. The engine and the dashboard's Your sky, with the tokens, the
   setting and the contrast check (the next issue).
2. Signed-out home and Explore (the same scene, public data).
3. Sign in and register (the mark).
4. Diffusion ripples.
5. The swarm on Admin › Runners › Images.
6. Adrift on error pages.

## 9. Open questions

- Should a cluster's lit marks show the event's title on rest, as a
  tooltip would? It is useful, but it is more text in the background.
  For now, no: the activity list beside them has the titles.
- Organisation pages could have their own sky (the organisation's
  repositories). That is cheap once Your sky exists.