From cd42406850e7ed33e47feb7d6590212106648d88 Mon Sep 17 00:00:00 2001
From: kolaente <k@knt.li>
Date: Mon, 2 Mar 2026 09:17:25 +0100
Subject: [PATCH] feat(gantt): create arrow SVG overlay component for relations

---
 .../components/gantt/GanttRelationArrows.vue  | 131 ++++++++++++++++++
 1 file changed, 131 insertions(+)
 create mode 100644 frontend/src/components/gantt/GanttRelationArrows.vue

diff --git a/frontend/src/components/gantt/GanttRelationArrows.vue b/frontend/src/components/gantt/GanttRelationArrows.vue
new file mode 100644
index 000000000..a01a92b93
--- /dev/null
+++ b/frontend/src/components/gantt/GanttRelationArrows.vue
@@ -0,0 +1,131 @@
+<template>
+	<svg
+		class="gantt-relation-arrows"
+		:width="width"
+		:height="height"
+		xmlns="http://www.w3.org/2000/svg"
+		aria-hidden="true"
+	>
+		<defs>
+			<marker
+				id="arrowhead-danger"
+				markerWidth="8"
+				markerHeight="8"
+				refX="6"
+				refY="4"
+				orient="auto"
+			>
+				<polygon
+					points="0,0 8,4 0,8"
+					fill="var(--danger)"
+				/>
+			</marker>
+			<marker
+				id="arrowhead-grey"
+				markerWidth="8"
+				markerHeight="8"
+				refX="6"
+				refY="4"
+				orient="auto"
+			>
+				<polygon
+					points="0,0 8,4 0,8"
+					fill="var(--grey-500)"
+				/>
+			</marker>
+		</defs>
+
+		<path
+			v-for="(arrow, index) in arrows"
+			:key="`arrow-${index}`"
+			:d="computePath(arrow)"
+			:stroke="arrow.color"
+			stroke-width="2"
+			fill="none"
+			:marker-end="getMarkerEnd(arrow)"
+			class="gantt-arrow"
+		/>
+	</svg>
+</template>
+
+<script setup lang="ts">
+import type {GanttArrow} from '@/helpers/ganttRelationArrows'
+
+defineProps<{
+	arrows: GanttArrow[]
+	width: number
+	height: number
+	rowHeight: number
+}>()
+
+/**
+ * Computes a bezier curve path for an arrow.
+ * Uses horizontal bezier curves that curve around obstacles.
+ */
+function computePath(arrow: GanttArrow): string {
+	const {startX, startY, endX, endY} = arrow
+
+	// Horizontal distance
+	const dx = endX - startX
+	const dy = endY - startY
+
+	// Control point offset (how much the curve bends)
+	const cpOffset = Math.min(Math.abs(dx) * 0.4, 60)
+
+	if (dx >= 0) {
+		// Target is to the right - simple S-curve
+		const cp1x = startX + cpOffset
+		const cp1y = startY
+		const cp2x = endX - cpOffset
+		const cp2y = endY
+
+		return `M ${startX} ${startY} C ${cp1x} ${cp1y}, ${cp2x} ${cp2y}, ${endX} ${endY}`
+	} else {
+		// Target is to the left - need to route around
+		// Go right first, then down/up, then left to target
+		const routeOffset = 30
+		const midY = startY + (dy > 0 ? routeOffset : -routeOffset)
+
+		const cp1x = startX + routeOffset
+		const cp1y = startY
+		const cp2x = startX + routeOffset
+		const cp2y = midY
+
+		const cp3x = endX - routeOffset
+		const cp3y = midY
+		const cp4x = endX - routeOffset
+		const cp4y = endY
+
+		return `M ${startX} ${startY} ` +
+			`C ${cp1x} ${cp1y}, ${cp2x} ${cp2y}, ${startX + routeOffset} ${midY} ` +
+			`L ${endX - routeOffset} ${midY} ` +
+			`C ${cp3x} ${cp3y}, ${cp4x} ${cp4y}, ${endX} ${endY}`
+	}
+}
+
+function getMarkerEnd(arrow: GanttArrow): string {
+	return arrow.relationKind === 'blocking'
+		? 'url(#arrowhead-danger)'
+		: 'url(#arrowhead-grey)'
+}
+</script>
+
+<style scoped lang="scss">
+.gantt-relation-arrows {
+	position: absolute;
+	inset-block-start: 0;
+	inset-inline-start: 0;
+	pointer-events: none;
+	z-index: 3;
+}
+
+.gantt-arrow {
+	opacity: 0.7;
+	transition: opacity 0.2s ease;
+
+	&:hover {
+		opacity: 1;
+	}
+}
+</style>
+