app/frontend/src/components/Flow/flow.css

/*
 * React Flow, routed through the design tokens.
 *
 * Scoped to `.react-flow` rather than added to index.css, so the canvas needs no
 * tokens of its own. The `colorMode` prop is passed all the same: React Flow
 * stamps it on the wrapper as a class, and `.light` / `.dark` are the app's own
 * token scopes — an unset one defaults to `light` and re-themes the whole canvas
 * inside a dark shell. See the root DESIGN-GUIDELINES.md → Shells.
 */

.react-flow {
  --xy-background-color: var(--background);
  --xy-background-pattern-color: color-mix(
    in srgb,
    var(--muted-foreground) 30%,
    transparent
  );
  --xy-edge-stroke: color-mix(
    in srgb,
    var(--muted-foreground) 55%,
    transparent
  );
  --xy-edge-stroke-selected: var(--primary);
  --xy-edge-stroke-width: 1.5;
  /*
   * What a pulse widens to on its way in; the width above is where it settles
   * back. Per canvas, because the brain draws its connections heavier.
   */
  --edge-pulse-width: 2.5;
  /*
   * The line dragged between two ports is the edge it is about to become: the
   * selected stroke at the same weight, so nothing shifts on release.
   */
  --xy-connectionline-stroke: var(--primary);
  --xy-connectionline-stroke-width: var(--xy-edge-stroke-width);
  --xy-handle-background-color: var(--card);
  --xy-handle-border-color: var(--muted-foreground);
  --xy-selection-background-color: color-mix(
    in srgb,
    var(--primary) 8%,
    transparent
  );
  --xy-selection-border: 1px solid var(--primary);
  --xy-attribution-background-color: transparent;
}

/*
 * Where a pulse lands: the opaque twin of the stroke the edge rests in, so the
 * last step back is invisible. A selected edge rests in blue and so decays into
 * it rather than crossing to grey and snapping back at the end.
 */
.react-flow__edge {
  --edge-rest: var(--muted-foreground);
}

.react-flow__edge.selected {
  --edge-rest: var(--primary);
}

/*
 * The dashes are the signal, the march below is only its motion: the pattern
 * is declared outside the guard so a reduced-motion reader still sees which
 * edges stream.
 */
.react-flow__edge-path.edge-stream {
  stroke-dasharray: 6 4;
}

/* A message arriving lights its edge, then decays back to rest. */
@media (prefers-reduced-motion: no-preference) {
  .react-flow__edge-path.edge-live {
    animation: edge-pulse var(--duration-pulse) var(--ease-emphasized);
  }

  /*
   * Both ends are plain colours. The implied `to` would be the edge's resting
   * stroke, which is a `color-mix()` — and interpolating a hex against one of
   * those goes through oklab and leaves the gamut on the way, which turned
   * every pulse fluorescent yellow.
   */
  @keyframes edge-pulse {
    from {
      stroke: var(--primary);
      stroke-width: var(--edge-pulse-width);
    }
    to {
      stroke: var(--edge-rest);
      stroke-width: var(--xy-edge-stroke-width);
    }
  }

  /*
   * An output that publishes repeatedly during one run reads as a march rather
   * than a line. Only the dash offset moves, so the pulse above still shows
   * through on the same path: it recolours and widens the dashes as they go.
   * A loop rather than a one-shot, so none of the `--duration-*` tokens fit —
   * the literal is one dash period per step, slow enough to read as direction.
   */
  .react-flow__edge-path.edge-stream {
    animation: edge-march 700ms linear infinite;
  }

  /* Both at once: one `animation` declaration has to carry both, or the
     cascade keeps whichever rule came last and drops the other. */
  .react-flow__edge-path.edge-stream.edge-live {
    animation:
      edge-pulse var(--duration-pulse) var(--ease-emphasized),
      edge-march 700ms linear infinite;
  }

  @keyframes edge-march {
    to {
      stroke-dashoffset: -10;
    }
  }
}

/*
 * A node that just published something says so, once, and settles: an arc runs
 * one lap of the rim and goes out. The same arc, lapping, is a node still at
 * work.
 *
 * The angle has to be registered to be animatable at all — an unregistered
 * custom property is a token stream, and swapping one for another mid-gradient
 * would step rather than sweep. Declared outside the guard below because it is
 * a type, not motion.
 */
@property --node-pulse-angle {
  syntax: "<angle>";
  inherits: false;
  initial-value: 0turn;
}

@media (prefers-reduced-motion: no-preference) {
  /*
   * Hangs off the wrapper the node's border is *not* on: `inset` on an absolute
   * child resolves against the padding box, so a ring measured from the node
   * itself lands inside a thick border — the brain's status ring is one — and
   * paints over it. Measured from the wrapper, the same offsets clear the rim
   * whatever border the node draws. The wrapper carries the radius too, which
   * is what lets one rule fit both a rounded card and a circle.
   *
   * The band is the padding: the content box is cut back out of the mask, which
   * leaves a ring rather than the filled shape the gradient paints. Both are
   * overridable, for a node whose own bands say where the arc belongs — the
   * brain's neuron runs it in the gap between its ring and its disc.
   */
  .node-pulse {
    position: absolute;
    inset: var(--node-pulse-inset, -4px);
    border-radius: inherit;
    padding: var(--node-pulse-band, 2px);
    pointer-events: none;
    background: conic-gradient(
      from var(--node-pulse-angle),
      transparent,
      var(--primary) 90deg,
      transparent 90deg
    );
    mask:
      linear-gradient(#000 0 0) content-box exclude,
      linear-gradient(#000 0 0);
    animation: node-pulse var(--duration-pulse) linear forwards;
  }

  /*
   * One lap at a constant rate, because the arc is travelling: an ease would
   * read as braking into the finish. The element stays mounted until the next
   * emit remounts it, so the lap has to put itself out on the way past.
   */
  @keyframes node-pulse {
    from {
      --node-pulse-angle: 0turn;
    }
    75% {
      opacity: 1;
    }
    to {
      --node-pulse-angle: 1turn;
      opacity: 0;
    }
  }

  /*
   * A node still working keeps the arc going instead: a run lasts as long as it
   * lasts — a training is minutes — so the lap repeats and never fades out, and
   * the ring is taken down when the node stops rather than by the animation.
   * A loop rather than one-shot feedback, so none of the `--duration-*` tokens
   * fit; the literal is one lap slow enough to read as travel, like the edge
   * march above.
   */
  .node-pulse.node-pulse-run {
    animation: node-chase 1.2s linear infinite;
  }

  @keyframes node-chase {
    to {
      --node-pulse-angle: 1turn;
    }
  }
}

/* Handles are neutral: the one brand-secondary affordance here is Run. */
.react-flow__handle {
  width: 12px;
  height: 12px;
  border-width: 2px;
}

.react-flow__handle.unbound {
  border-style: dashed;
  border-color: color-mix(in srgb, var(--muted-foreground) 40%, transparent);
}

.react-flow__handle-connecting {
  border-color: var(--primary);
}

.react-flow__node:focus-visible,
.react-flow__node:focus {
  outline: none;
}

/*
 * A keyboard has to see where it is. The outline is off above by design, so
 * focus is answered the way a neuron answers it — by recolouring the node's own
 * rim rather than by adding a ring around it. `--ring` equals `--primary`.
 */
.react-flow__node:focus-visible .flow-node-card {
  border-color: var(--ring);
}

/*
 * Brain graph. A neuron meets its connections all round its rim, so these two
 * handles are only there to make React Flow treat the node as wired at all —
 * where a connection actually lands is `BrainEdge`'s own geometry, measured
 * from the centre they sit at.
 */
.brain-handle {
  opacity: 0;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
}

/*
 * Embedded on Home rather than on a canvas of its own: the graph paints no
 * surface, so the page's own background shows through behind the neurons.
 *
 * `--brain-decay` is how long a neuron or a connection takes to dim from its
 * brightest back to the floor. Not a UI transition but the falloff of a signal,
 * so it is measured in minutes rather than in the `--duration-*` tokens.
 */
.react-flow.brain-flat {
  --xy-background-color: transparent;
  --brain-decay: 150s;
  /*
   * Thicker than the editor's, to match the weight the mark draws them at — and
   * the pulse widens with it, or a message would leave its edge looking thinner
   * than it rests.
   */
  --xy-edge-stroke-width: 3;
  --edge-pulse-width: 4;
  --xy-background-pattern-color: color-mix(
    in srgb,
    var(--muted-foreground) 22%,
    transparent
  );
}

/*
 * The ground the mark's nodes sit on. The band has no surface of its own and no
 * border, so a dot grid that ran to the edges would draw the rectangle the band
 * otherwise does not have. An elliptical mask fades it out well before any
 * edge — the same shape the layout settles into, so the dots run out roughly
 * where the graph does. The stops are the mask's alpha, not a colour: opaque
 * keeps the dots, transparent drops them.
 */
.brain-flat .react-flow__background {
  mask-image: radial-gradient(
    ellipse 62% 62% at 50% 50%,
    black 0%,
    transparent 100%
  );
}

/*
 * A neuron is the node of the brand mark: a disc of the identity blue, a gap in
 * the surface colour, then a thick ring. Ring and gap are shares of the
 * diameter and live in `BrainNode`, which is where the diameter is known; the
 * gap is what the mark's tile shows between the two, which here is the card the
 * band sits flat on.
 *
 * The two parts carry the two faults, the way the mark is built: the ring is
 * the wiring, so a flow that cannot run as written turns it `--brand-secondary`;
 * the disc is the node, so a run that broke turns the fill the same terracotta.
 * Both at once still reads, since the gap keeps them apart. This is the one
 * place terracotta means fault rather than a call to action — see the root
 * DESIGN-GUIDELINES.md → Colour tokens.
 *
 * What the fill's lightness carries is how recently this page saw the neuron
 * publish: brightest just after it fires, decaying to a floor it never drops
 * below, so a graph left open sorts itself into what is busy and what is not.
 * The floor stays saturated enough to still read as the mark. A fresh page has
 * seen nothing either way, so everything starts at `.brain-idle`, halfway,
 * claiming no history it does not have.
 */
.brain-cell {
  --brain-fill: var(--primary);
  --brain-gap: var(--card);
  cursor: pointer;
  background-color: color-mix(in srgb, var(--brain-fill) 50%, transparent);
  transition: border-color var(--duration-base) var(--ease-standard);
}

.brain-cell.brain-fault {
  --brain-fill: var(--brand-secondary);
}

.brain-cell.brain-idle {
  background-color: color-mix(in srgb, var(--brain-fill) 65%, transparent);
}

/* Snapped into, decayed out of — never the other way round. */
.brain-cell.brain-hot {
  background-color: color-mix(in srgb, var(--brain-fill) 90%, transparent);
  transition: none;
}

@media (prefers-reduced-motion: no-preference) {
  .brain-cell {
    transition:
      border-color var(--duration-base) var(--ease-standard),
      background-color var(--brain-decay) linear;
  }

  /*
   * A fault is a state change, not a falloff. Sharing the decay's transition
   * would have the disc creep from blue to terracotta over a minute and a half,
   * so the fill takes the same 200ms the ring beside it does.
   */
  .brain-cell.brain-fault {
    transition:
      border-color var(--duration-base) var(--ease-standard),
      background-color var(--duration-base) var(--ease-standard);
  }
}

/*
 * The pointer lights the gap rather than adding a fourth band around a circle
 * that already has three. Ring and disc both carry a status, so neither can be
 * recoloured to mark a hover; the surface between them carries nothing.
 */
.brain-cell:hover,
.react-flow__node:focus-visible .brain-cell {
  --brain-gap: var(--ring);
}

/*
 * A name under every circle is what makes the graph unreadable, so only the
 * hovered or focused neuron carries one. Absolutely positioned: the layout's
 * collide radius already holds this space clear, and revealing a label must not
 * move anything.
 */
.brain-label {
  position: absolute;
  top: 100%;
  left: 50%;
  transform: translateX(-50%);
  margin-top: 6px;
  display: flex;
  flex-direction: column;
  align-items: center;
  visibility: hidden;
}

.brain-cell:hover .brain-label,
.react-flow__node:focus-visible .brain-label,
.brain-label.brain-label-on {
  visibility: visible;
}

/*
 * The same falloff as the neurons, on the connection instead of the disc. It
 * carries the line and its two end dots alike, so the ink is a variable on the
 * group they share rather than a stroke on each.
 *
 * Drawn from `--foreground`, the same token as the rings: in the mark a
 * connection is the same stroke as the ring it runs into, and mixing the two
 * families instead left the graph looking like rims in one drawing and wires
 * from another.
 */
.brain-wire {
  --brain-ink: color-mix(in srgb, var(--foreground) 16%, transparent);
}

.brain-wire.brain-idle {
  --brain-ink: color-mix(in srgb, var(--foreground) 48%, transparent);
}

/* Snapped into, decayed out of — never the other way round. */
.brain-wire.brain-hot {
  --brain-ink: color-mix(in srgb, var(--foreground) 80%, transparent);
}

.react-flow__edge-path.brain-edge {
  stroke: var(--brain-ink);
  stroke-linecap: round;
  stroke-linejoin: round;
}

/*
 * Where a connection meets a neuron: a flow node's port, sat on the rim the same
 * way, but told apart by direction — filled at the end the value leaves, hollow
 * at the end it arrives. The hollow one takes the surface the band sits flat on,
 * so it reads as a hole rather than as a disc of its own.
 */
.brain-dot {
  fill: var(--card);
  stroke: var(--brain-ink);
  stroke-width: 2;
}

.brain-dot.brain-dot-out {
  fill: var(--brain-ink);
}

@media (prefers-reduced-motion: no-preference) {
  .react-flow__edge-path.brain-edge,
  .brain-dot {
    transition:
      stroke var(--brain-decay) linear,
      fill var(--brain-decay) linear;
  }

  .brain-wire.brain-hot .react-flow__edge-path.brain-edge,
  .brain-wire.brain-hot .brain-dot {
    transition: none;
  }

  /*
   * A value passing lights the line for the pulse and then decays; the dots sit
   * on the same signal, so they flash with it instead of only following the
   * falloff — same duration and easing as `edge-pulse`, and the same plain
   * `--edge-rest` at the far end rather than the `color-mix()` the group rests
   * in, which is what turned an interpolated pulse fluorescent.
   *
   * Two keyframe sets, because an animation beats a normal declaration whatever
   * its specificity: animating `fill` for every dot would fill the hollow
   * arriving end for the length of the pulse, and that hole is what tells the
   * two ends apart.
   */
  .brain-wire.brain-hot .brain-dot {
    animation: brain-dot-pulse var(--duration-pulse) var(--ease-emphasized);
  }

  .brain-wire.brain-hot .brain-dot.brain-dot-out {
    animation: brain-dot-out-pulse var(--duration-pulse) var(--ease-emphasized);
  }

  @keyframes brain-dot-pulse {
    from {
      stroke: var(--primary);
    }
    to {
      stroke: var(--edge-rest);
    }
  }

  @keyframes brain-dot-out-pulse {
    from {
      stroke: var(--primary);
      fill: var(--primary);
    }
    to {
      stroke: var(--edge-rest);
      fill: var(--edge-rest);
    }
  }
}