docs: Backdrops, living backgrounds specified (refs #156) 2f14c5f39e81

# Backdrops: living backgrounds for OxygenHub

Status: proposal, 2026-10-13 (oxygenhub#156). Nothing here is built yet;

the dashboard's scene comes first.

## 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 22% | `--acc` at 18% |

| `--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: changes of the same

  repository, or a change and the one it depends on where the scene has

  dependencies. 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 beside it 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:

- one record per repository you can see on the dashboard: its name and

  its activity count in the last 30 days;

- up to 400 events from the last 30 days across those repositories

  (pushes, proposals, issues, releases): each event's repository, kind

  and time.

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, placed by its name's BLAKE3 on a golden

  angle spiral, so your sky looks the same on every visit and on every

  device.

- The most active repositories sit nearest the middle 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 push is a dot, a proposal is a

  small diamond, an issue is a ring and a release is a four-point star.

**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

One script, `assets/backdrop.js`, an ES module of about 15 KB with no

build step, served by the hub like `hub.js`:

- **Drawing.** WebGL2 with instanced quads for marks (one draw for all)

  and one line buffer for links. If WebGL2 is unavailable, it falls back

  to Canvas 2D; if that fails too, 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 not Rust and WASM?** The rest of Oxygen is Rust, so it is a fair

question. But a scene of a few hundred marks is light work for a small

script, and WASM would add a second toolchain to the hub build and to

the offline CI image (wasm32 target, wasm-bindgen, web-sys vendored) for

no visible gain. What makes these backdrops new is that they show real

data. Revisit WASM if a scene ever needs heavy layout, such as a

repository's full change graph of thousands of changes, where sharing

oxy-core's own types would pay.

## 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.