From 3b76025e10c9ae4dc06e294e2a4b701536260cf1 Mon Sep 17 00:00:00 2001
From: Hare <kei.hiracchi.0928@gmail.com>
Date: Sat, 10 Oct 2026 21:01:06 +0900
Subject: [PATCH] fix: anchor trunk origin to the slash scale

---
 site/src/scripts/landing.ts        |  4 +++-
 site/tests/browser/landing.spec.ts | 16 ++++++++++++----
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/site/src/scripts/landing.ts b/site/src/scripts/landing.ts
index d89ed09..c8b53df 100644
--- a/site/src/scripts/landing.ts
+++ b/site/src/scripts/landing.ts
@@ -263,7 +263,9 @@ export function initLanding() {
     const extension = timing.trace(progress(time, flow.grow, 1000));
     const converge = timing.travel(progress(time, flow.converge, 560));
     const longLength = Math.max(0, innerHeight * .4 - root.offsetHeight * 7.2 / 2 - 8);
-    const lineStartY = centreY + root.offsetHeight * scale / 2 + 8 * (1 - converge);
+    // Anchor to the root's local bottom edge at every scale. Only the tip
+    // extends/converges; the origin has no independent screen-space offset.
+    const lineStartY = centreY + root.offsetHeight * scale / 2;
     const finalLength = lastRootBranch.offsetTop + lastRootBranch.offsetHeight / 2 - slot.offsetTop - slot.offsetHeight;
     const tipY = lineStartY + mix(longLength * extension, finalLength * scale, converge);
     const handoff = progress(time, flow.handoff, 280);
diff --git a/site/tests/browser/landing.spec.ts b/site/tests/browser/landing.spec.ts
index 8e4964c..e93e0fb 100644
--- a/site/tests/browser/landing.spec.ts
+++ b/site/tests/browser/landing.spec.ts
@@ -89,7 +89,7 @@ test('hover streams are masked at the actual slash edges throughout the loop', a
   await expect(page.locator('.tap-stream-b')).toHaveCSS('opacity', '0');
 });
 
-test('trunk thickness keeps the same proportion to the slash throughout scaling', async ({ page }, testInfo) => {
+test('trunk thickness and origin stay proportional to the slash throughout scaling', async ({ page }, testInfo) => {
   await page.clock.install({ time: new Date('2026-01-01T00:00:00Z') });
   await page.clock.pauseAt(new Date('2026-01-01T00:00:01Z'));
   await armed(page);
@@ -102,12 +102,20 @@ test('trunk thickness keeps the same proportion to the slash throughout scaling'
       const path = document.querySelector<SVGPathElement>('.trunk-line')!;
       const matrix = path.getScreenCTM()!;
       const stroke = parseFloat(getComputedStyle(path).strokeWidth) * Math.hypot(matrix.a, matrix.b);
-      const slashHeight = document.querySelector('.root-shape')!.getBoundingClientRect().height;
-      return { stroke, slashHeight, opacity: Number(getComputedStyle(path).opacity) };
+      const slash = document.querySelector('.root-shape')!.getBoundingClientRect();
+      const start = path.getPointAtLength(0).matrixTransform(matrix);
+      return {
+        stroke, slashHeight: slash.height, opacity: Number(getComputedStyle(path).opacity),
+        anchorX: (start.x - slash.left) / slash.width,
+        gapRatio: (start.y - slash.bottom) / slash.height,
+      };
     });
     expect(ink.opacity).toBeGreaterThan(0);
     // At rest: 1px trunk, 21px of actual slash ink (56/64 of its 24px box).
     expect(ink.stroke / ink.slashHeight).toBeCloseTo(1 / 21, 5);
+    // The same local anchor at every scale: centred, 1.5px below 21px of ink.
+    expect(ink.anchorX).toBeCloseTo(.5, 5);
+    expect(ink.gapRatio).toBeCloseTo(1 / 14, 5);
     expect(ink.stroke).toBeLessThan(previousWidth);
     if ([1520, 2000, 2480].includes(time)) await page.screenshot({ path: testInfo.outputPath(`proportions-${time}.png`) });
     previousTime = time;
@@ -194,7 +202,7 @@ test('continuous motion overlaps growth, travel and reveal without intermediate
   expect(Math.abs(extended.dx)).toBeLessThan(.1);
   expect(Math.abs(extended.axis)).toBeLessThan(.1);
   expect(extended.dy).toBeGreaterThan(0);
-  expect(extended.gap).toBeCloseTo(8, 0);
+  expect(extended.gap).toBeCloseTo(0, 1);
   expect(extended.bottom).toBeLessThan(page.viewportSize()!.height);
   await page.screenshot({ path: testInfo.outputPath('04-travelling-trunk.png') });
   await page.clock.runFor(920); // 2600ms: moving tree, SVG handoff AND panel onset.