feat: drag relation tags along threads

src/boardDomain.test.ts

@@ -11,7 +11,10 @@ import {
   normalizeCase,
   panViewport,
   relationPosition,
+  projectThreadTag,
   threadCurve,
+  threadTagLateralLimit,
+  threadTagPlacement,
   timelinePositionPercent,
   timelineRange,
   zoomFromWheel,
@@ -31,6 +34,18 @@ describe('red thread geometry', () => {
   it('clamps tightness to its normalized percentage range', () => {
     expect(threadCurve({ x: 0, y: 0 }, { x: 100, y: 0 }, 200)).toEqual(threadCurve({ x: 0, y: 0 }, { x: 100, y: 0 }, 100))
   })
+
+  it('places and projects tags by percentage along the curve', () => {
+    const from = { x: 0, y: 0 }, to = { x: 200, y: 0 }
+    expect(threadTagPlacement(from, to, 100, 25, 0)).toMatchObject({ x: 50, y: 0, positionPercent: 25 })
+    expect(projectThreadTag(from, to, 100, { x: 150, y: 8 })).toMatchObject({ positionPercent: 75, lateralOffset: 8 })
+  })
+
+  it('reduces lateral tag travel as the thread becomes taut', () => {
+    expect(threadTagLateralLimit(0)).toBe(70)
+    expect(threadTagLateralLimit(100)).toBe(10)
+    expect(threadTagPlacement({ x: 0, y: 0 }, { x: 200, y: 0 }, 100, 50, 100).lateralOffset).toBe(10)
+  })
 })
 
 const folder: Evidence = {