From 36701feb9ecb14ab79c6b5ceff0941d50925078d Mon Sep 17 00:00:00 2001
From: cottongin <cottongin@users.noreply.github.com>
Date: Mon, 21 Sep 2026 11:03:24 -0400
Subject: [PATCH] feat: add canvas-based timeline with playhead, scrubbing, and
 zoom

Co-authored-by: Cursor <cursoragent@cursor.com>
---
 src/lib/components/Timeline.svelte      | 134 ++++++++++++++++++++++--
 src/lib/timeline/interactions.ts        |  48 +++++++++
 src/lib/timeline/renderer.ts            | 128 ++++++++++++++++++++++
 tests/lib/timeline/interactions.test.ts |  40 +++++++
 tests/lib/timeline/renderer.test.ts     |  57 ++++++++++
 5 files changed, 398 insertions(+), 9 deletions(-)
 create mode 100644 src/lib/timeline/interactions.ts
 create mode 100644 src/lib/timeline/renderer.ts
 create mode 100644 tests/lib/timeline/interactions.test.ts
 create mode 100644 tests/lib/timeline/renderer.test.ts

diff --git a/src/lib/components/Timeline.svelte b/src/lib/components/Timeline.svelte
index d1437b9..05c6f65 100644
--- a/src/lib/components/Timeline.svelte
+++ b/src/lib/components/Timeline.svelte
@@ -1,17 +1,133 @@
-<div class="timeline">
-  <div class="placeholder-text">Timeline</div>
+<script lang="ts">
+  import { session } from '$lib/stores/videoSession.svelte';
+  import { drawTimeline, type TimelineState } from '$lib/timeline/renderer';
+  import {
+    computeZoom,
+    handleClick as computeClickTime,
+  } from '$lib/timeline/interactions';
+
+  let canvas = $state<HTMLCanvasElement | null>(null);
+  let containerEl = $state<HTMLDivElement | null>(null);
+  let animFrameId = $state(0);
+
+  let timelineState = $state<TimelineState>({
+    visibleStart: 0,
+    visibleEnd: 60,
+    zoom: 1,
+    width: 600,
+    height: 160,
+  });
+
+  let isDragging = $state(false);
+
+  $effect(() => {
+    if (session.duration > 0) {
+      timelineState.visibleEnd = session.duration;
+    }
+  });
+
+  $effect(() => {
+    if (!containerEl) return;
+    const observer = new ResizeObserver((entries) => {
+      for (const entry of entries) {
+        timelineState.width = entry.contentRect.width;
+        timelineState.height = entry.contentRect.height;
+        if (canvas) {
+          canvas.width = entry.contentRect.width * window.devicePixelRatio;
+          canvas.height = entry.contentRect.height * window.devicePixelRatio;
+          canvas.style.width = `${entry.contentRect.width}px`;
+          canvas.style.height = `${entry.contentRect.height}px`;
+        }
+      }
+    });
+    observer.observe(containerEl);
+    return () => observer.disconnect();
+  });
+
+  $effect(() => {
+    if (!canvas) return;
+    const ctx = canvas.getContext('2d');
+    if (!ctx) return;
+
+    function draw() {
+      if (!ctx || !canvas) return;
+      ctx.save();
+      ctx.scale(window.devicePixelRatio, window.devicePixelRatio);
+      drawTimeline(ctx, timelineState, session.currentTime, session.duration);
+      ctx.restore();
+      animFrameId = requestAnimationFrame(draw);
+    }
+
+    draw();
+    return () => cancelAnimationFrame(animFrameId);
+  });
+
+  function getCanvasX(e: MouseEvent): number {
+    if (!canvas) return 0;
+    const rect = canvas.getBoundingClientRect();
+    return e.clientX - rect.left;
+  }
+
+  function seekTo(time: number) {
+    session.currentTime = time;
+    const videoEl = document.querySelector('video');
+    if (videoEl) {
+      videoEl.currentTime = time;
+    }
+  }
+
+  function handleMouseDown(e: MouseEvent) {
+    isDragging = true;
+    const x = getCanvasX(e);
+    const targetTime = computeClickTime(x, timelineState, session.duration);
+    seekTo(targetTime);
+  }
+
+  function handleMouseMove(e: MouseEvent) {
+    if (!isDragging) return;
+    const x = getCanvasX(e);
+    const targetTime = computeClickTime(x, timelineState, session.duration);
+    seekTo(targetTime);
+  }
+
+  function handleMouseUp() {
+    isDragging = false;
+  }
+
+  function handleWheel(e: WheelEvent) {
+    e.preventDefault();
+    const x = getCanvasX(e);
+    const anchorTime = computeClickTime(x, timelineState, session.duration);
+    const factor = e.deltaY < 0 ? 1.15 : 0.87;
+    const result = computeZoom(timelineState, anchorTime, factor, session.duration);
+    timelineState.visibleStart = result.visibleStart;
+    timelineState.visibleEnd = result.visibleEnd;
+    timelineState.zoom = result.zoom;
+  }
+</script>
+
+<div class="timeline-container" bind:this={containerEl}>
+  <canvas
+    bind:this={canvas}
+    onmousedown={handleMouseDown}
+    onmousemove={handleMouseMove}
+    onmouseup={handleMouseUp}
+    onmouseleave={handleMouseUp}
+    onwheel={handleWheel}
+  ></canvas>
 </div>
 
 <style>
-  .timeline {
+  .timeline-container {
     height: 160px;
     background: var(--timeline-bg);
-    display: flex;
-    align-items: center;
-    justify-content: center;
+    position: relative;
+    cursor: crosshair;
   }
-  .placeholder-text {
-    color: var(--text-muted);
-    font-size: 13px;
+
+  canvas {
+    display: block;
+    width: 100%;
+    height: 100%;
   }
 </style>
diff --git a/src/lib/timeline/interactions.ts b/src/lib/timeline/interactions.ts
new file mode 100644
index 0000000..75b7439
--- /dev/null
+++ b/src/lib/timeline/interactions.ts
@@ -0,0 +1,48 @@
+import { type TimelineState, xToTime } from './renderer';
+
+export type { TimelineState };
+
+const MIN_VISIBLE_RANGE = 0.1;
+const MIN_ZOOM = 1;
+
+export function clampZoom(zoom: number, duration: number): number {
+  const maxZoom = duration / MIN_VISIBLE_RANGE;
+  return Math.max(MIN_ZOOM, Math.min(zoom, maxZoom));
+}
+
+export function computeZoom(
+  state: TimelineState,
+  anchorTime: number,
+  factor: number,
+  duration: number
+): { visibleStart: number; visibleEnd: number; zoom: number } {
+  const currentRange = state.visibleEnd - state.visibleStart;
+  const newRange = currentRange / factor;
+  const clampedRange = Math.max(MIN_VISIBLE_RANGE, Math.min(newRange, duration));
+
+  const anchorRatio = (anchorTime - state.visibleStart) / currentRange;
+  let newStart = anchorTime - anchorRatio * clampedRange;
+  let newEnd = newStart + clampedRange;
+
+  if (newStart < 0) {
+    newStart = 0;
+    newEnd = clampedRange;
+  }
+  if (newEnd > duration) {
+    newEnd = duration;
+    newStart = Math.max(0, newEnd - clampedRange);
+  }
+
+  const newZoom = clampZoom(duration / clampedRange, duration);
+
+  return { visibleStart: newStart, visibleEnd: newEnd, zoom: newZoom };
+}
+
+export function handleClick(
+  x: number,
+  state: TimelineState,
+  duration: number
+): number {
+  const time = xToTime(x, state);
+  return Math.max(0, Math.min(time, duration));
+}
diff --git a/src/lib/timeline/renderer.ts b/src/lib/timeline/renderer.ts
new file mode 100644
index 0000000..d4b8f41
--- /dev/null
+++ b/src/lib/timeline/renderer.ts
@@ -0,0 +1,128 @@
+export interface TimelineState {
+  visibleStart: number;
+  visibleEnd: number;
+  zoom: number;
+  width: number;
+  height: number;
+}
+
+export function timeToX(time: number, state: TimelineState): number {
+  const range = state.visibleEnd - state.visibleStart;
+  if (range <= 0) return 0;
+  return ((time - state.visibleStart) / range) * state.width;
+}
+
+export function xToTime(x: number, state: TimelineState): number {
+  const range = state.visibleEnd - state.visibleStart;
+  return state.visibleStart + (x / state.width) * range;
+}
+
+const THUMB_LANE_HEIGHT = 50;
+const WAVEFORM_LANE_HEIGHT = 40;
+const CONTROLS_LANE_HEIGHT = 30;
+const TICK_HEIGHT = 10;
+
+export function drawTimeline(
+  ctx: CanvasRenderingContext2D,
+  state: TimelineState,
+  currentTime: number,
+  duration: number
+): void {
+  const { width, height } = state;
+  ctx.clearRect(0, 0, width, height);
+
+  ctx.fillStyle = '#181825';
+  ctx.fillRect(0, 0, width, height);
+
+  drawTimeTicks(ctx, state, duration);
+
+  drawPlaceholderLane(ctx, state, 0, THUMB_LANE_HEIGHT, 'Thumbnails');
+  drawPlaceholderLane(ctx, state, THUMB_LANE_HEIGHT, WAVEFORM_LANE_HEIGHT, 'Waveform');
+
+  drawPlayhead(ctx, state, currentTime);
+}
+
+function drawTimeTicks(
+  ctx: CanvasRenderingContext2D,
+  state: TimelineState,
+  duration: number
+): void {
+  const range = state.visibleEnd - state.visibleStart;
+  const tickInterval = getTickInterval(range, state.width);
+  const startTick = Math.floor(state.visibleStart / tickInterval) * tickInterval;
+
+  ctx.fillStyle = '#6c7086';
+  ctx.font = '10px -apple-system, sans-serif';
+  ctx.textAlign = 'center';
+
+  for (let t = startTick; t <= state.visibleEnd; t += tickInterval) {
+    if (t < 0 || t > duration) continue;
+    const x = timeToX(t, state);
+    ctx.strokeStyle = '#45475a';
+    ctx.beginPath();
+    ctx.moveTo(x, state.height - CONTROLS_LANE_HEIGHT);
+    ctx.lineTo(x, state.height - CONTROLS_LANE_HEIGHT + TICK_HEIGHT);
+    ctx.stroke();
+
+    const label = formatTickLabel(t);
+    ctx.fillText(label, x, state.height - CONTROLS_LANE_HEIGHT + TICK_HEIGHT + 12);
+  }
+}
+
+function getTickInterval(visibleRange: number, width: number): number {
+  const minPixelsPerTick = 80;
+  const maxTicks = width / minPixelsPerTick;
+  const rawInterval = visibleRange / maxTicks;
+
+  const intervals = [0.1, 0.25, 0.5, 1, 2, 5, 10, 15, 30, 60, 120, 300, 600];
+  for (const interval of intervals) {
+    if (interval >= rawInterval) return interval;
+  }
+  return 600;
+}
+
+function formatTickLabel(seconds: number): string {
+  const m = Math.floor(seconds / 60);
+  const s = Math.floor(seconds % 60);
+  return `${m}:${String(s).padStart(2, '0')}`;
+}
+
+function drawPlaceholderLane(
+  ctx: CanvasRenderingContext2D,
+  state: TimelineState,
+  y: number,
+  height: number,
+  label: string
+): void {
+  ctx.fillStyle = '#2a2a3e';
+  ctx.fillRect(0, y, state.width, height);
+  ctx.strokeStyle = '#353550';
+  ctx.strokeRect(0, y, state.width, height);
+  ctx.fillStyle = '#6c7086';
+  ctx.font = '11px -apple-system, sans-serif';
+  ctx.textAlign = 'center';
+  ctx.fillText(`Generating ${label}…`, state.width / 2, y + height / 2 + 4);
+}
+
+export function drawPlayhead(
+  ctx: CanvasRenderingContext2D,
+  state: TimelineState,
+  currentTime: number
+): void {
+  const x = timeToX(currentTime, state);
+  ctx.strokeStyle = '#ffffff';
+  ctx.lineWidth = 2;
+  ctx.beginPath();
+  ctx.moveTo(x, 0);
+  ctx.lineTo(x, state.height);
+  ctx.stroke();
+  ctx.lineWidth = 1;
+
+  ctx.fillStyle = '#ffffff';
+  ctx.beginPath();
+  ctx.moveTo(x - 5, 0);
+  ctx.lineTo(x + 5, 0);
+  ctx.lineTo(x, 8);
+  ctx.closePath();
+  ctx.fill();
+}
diff --git a/tests/lib/timeline/interactions.test.ts b/tests/lib/timeline/interactions.test.ts
new file mode 100644
index 0000000..e624454
--- /dev/null
+++ b/tests/lib/timeline/interactions.test.ts
@@ -0,0 +1,40 @@
+import { describe, it, expect } from 'vitest';
+import { clampZoom, computeZoom, type TimelineState } from '$lib/timeline/interactions';
+
+describe('clampZoom', () => {
+  it('clamps zoom above minimum', () => {
+    expect(clampZoom(0.5, 60)).toBeGreaterThanOrEqual(1);
+  });
+
+  it('clamps zoom below maximum', () => {
+    expect(clampZoom(10000, 60)).toBeLessThanOrEqual(600);
+  });
+});
+
+describe('computeZoom', () => {
+  it('zooming in narrows the visible range', () => {
+    const state: TimelineState = {
+      visibleStart: 0,
+      visibleEnd: 60,
+      zoom: 1,
+      width: 600,
+      height: 160,
+    };
+    const result = computeZoom(state, 30, 1.1, 60);
+    const newRange = result.visibleEnd - result.visibleStart;
+    expect(newRange).toBeLessThan(60);
+  });
+
+  it('zooming out widens the visible range', () => {
+    const state: TimelineState = {
+      visibleStart: 10,
+      visibleEnd: 20,
+      zoom: 6,
+      width: 600,
+      height: 160,
+    };
+    const result = computeZoom(state, 15, 0.9, 60);
+    const newRange = result.visibleEnd - result.visibleStart;
+    expect(newRange).toBeGreaterThan(10);
+  });
+});
diff --git a/tests/lib/timeline/renderer.test.ts b/tests/lib/timeline/renderer.test.ts
new file mode 100644
index 0000000..988ac07
--- /dev/null
+++ b/tests/lib/timeline/renderer.test.ts
@@ -0,0 +1,57 @@
+import { describe, it, expect } from 'vitest';
+import { timeToX, xToTime, type TimelineState } from '$lib/timeline/renderer';
+
+function makeState(overrides: Partial<TimelineState> = {}): TimelineState {
+  return {
+    visibleStart: 0,
+    visibleEnd: 60,
+    zoom: 1,
+    width: 600,
+    height: 160,
+    ...overrides,
+  };
+}
+
+describe('timeToX', () => {
+  it('maps start of visible range to 0', () => {
+    const state = makeState();
+    expect(timeToX(0, state)).toBe(0);
+  });
+
+  it('maps end of visible range to width', () => {
+    const state = makeState();
+    expect(timeToX(60, state)).toBe(600);
+  });
+
+  it('maps midpoint correctly', () => {
+    const state = makeState();
+    expect(timeToX(30, state)).toBe(300);
+  });
+
+  it('handles zoomed-in view', () => {
+    const state = makeState({ visibleStart: 10, visibleEnd: 20 });
+    expect(timeToX(15, state)).toBe(300);
+  });
+});
+
+describe('xToTime', () => {
+  it('maps 0 to visible start', () => {
+    const state = makeState();
+    expect(xToTime(0, state)).toBe(0);
+  });
+
+  it('maps width to visible end', () => {
+    const state = makeState();
+    expect(xToTime(600, state)).toBe(60);
+  });
+
+  it('maps midpoint correctly', () => {
+    const state = makeState();
+    expect(xToTime(300, state)).toBe(30);
+  });
+
+  it('inverse of timeToX', () => {
+    const state = makeState({ visibleStart: 10, visibleEnd: 20 });
+    expect(xToTime(timeToX(15, state), state)).toBeCloseTo(15);
+  });
+});