Add edge midpoint drag handles to Grid_Setup

Drag indices 4-7 correspond to top/right/bottom/left edge midpoints.
Dragging a midpoint applies the delta to both adjacent corners, making
it easier to align bins with rounded corners where corner handles may
be obscured.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/views/grid-setup.mjs

@@ -14,13 +14,17 @@ export class Grid_Setup {
 	#cam_z = 1;
 
 	#corners = null;   // in IMAGE coordinates
-	#drag_idx = -1;    // index of corner being dragged, or -1
+	#drag_idx = -1;    // 0-3: corners, 4-7: edge midpoints (top,right,bottom,left)
+	#drag_prev_img = null;  // previous image-space position for midpoint delta tracking
 	#panning = false;
 	#pan_last = { x: 0, y: 0 };
 
 	#rows = 4;
 	#cols = 6;
 
+	// Edge pairs for midpoint handles: midpoint (idx-4) moves corners [a, b]
+	static #MIDPOINT_EDGES = [[0,1],[1,2],[2,3],[3,0]];
+
 	constructor(canvas_el) {
 		this.#canvas = canvas_el;
 		this.#ctx = canvas_el.getContext('2d');
@@ -126,12 +130,26 @@ export class Grid_Setup {
 		return { x: w.x * this.#cam_z + this.#cam_x, y: w.y * this.#cam_z + this.#cam_y };
 	}
 
+	#get_midpoints() {
+		return Grid_Setup.#MIDPOINT_EDGES.map(([a, b]) => ({
+			x: (this.#corners[a].x + this.#corners[b].x) / 2,
+			y: (this.#corners[a].y + this.#corners[b].y) / 2,
+		}));
+	}
+
 	#find_handle(sp, radius = 18) {
 		if (!this.#corners) return -1;
+		// Corners take priority
 		for (let i = 0; i < 4; i++) {
 			const s = this.#img_to_screen(this.#corners[i]);
 			if ((sp.x - s.x)**2 + (sp.y - s.y)**2 < radius**2) return i;
 		}
+		// Midpoints
+		const mids = this.#get_midpoints();
+		for (let i = 0; i < 4; i++) {
+			const s = this.#img_to_screen(mids[i]);
+			if ((sp.x - s.x)**2 + (sp.y - s.y)**2 < (radius * 0.85)**2) return i + 4;
+		}
 		return -1;
 	}
 
@@ -141,10 +159,13 @@ export class Grid_Setup {
 		const hit = this.#find_handle(sp);
 		if (hit !== -1) {
 			this.#drag_idx = hit;
+			if (hit >= 4) {
+				this.#drag_prev_img = this.#world_to_img(this.#to_world(sp));
+			}
 		} else {
 			this.#panning = true;
 			this.#pan_last = sp;
-			if (!is_touch) this.#canvas.style.cursor = 'grabbing';
+			if (!is_touch) { this.#canvas.style.cursor = 'grabbing'; }
 		}
 	}
 
@@ -152,7 +173,16 @@ export class Grid_Setup {
 		const sp = this.#screen_pos(e);
 		if (this.#drag_idx !== -1) {
 			const img_pos = this.#world_to_img(this.#to_world(sp));
+			if (this.#drag_idx < 4) {
 				this.#corners[this.#drag_idx] = img_pos;
+			} else {
+				const [a, b] = Grid_Setup.#MIDPOINT_EDGES[this.#drag_idx - 4];
+				const dx = img_pos.x - this.#drag_prev_img.x;
+				const dy = img_pos.y - this.#drag_prev_img.y;
+				this.#corners[a] = { x: this.#corners[a].x + dx, y: this.#corners[a].y + dy };
+				this.#corners[b] = { x: this.#corners[b].x + dx, y: this.#corners[b].y + dy };
+				this.#drag_prev_img = img_pos;
+			}
 			this.#draw();
 		} else if (this.#panning) {
 			this.#cam_x += sp.x - this.#pan_last.x;
@@ -167,6 +197,7 @@ export class Grid_Setup {
 
 	#on_up(e) {
 		this.#drag_idx = -1;
+		this.#drag_prev_img = null;
 		if (this.#panning) {
 			this.#panning = false;
 			const sp = this.#screen_pos(e);
@@ -255,6 +286,19 @@ export class Grid_Setup {
 			ctx.fillText(LABELS[i], pt.x, pt.y);
 		});
 
+		// Midpoint handles — smaller, white with blue stroke
+		const mid_r = 7 / this.#cam_z;
+		const mids = this.#get_midpoints().map(m => this.#img_to_world(m));
+		mids.forEach(pt => {
+			ctx.beginPath();
+			ctx.arc(pt.x, pt.y, mid_r, 0, Math.PI*2);
+			ctx.fillStyle = 'rgba(255,255,255,0.75)';
+			ctx.fill();
+			ctx.strokeStyle = 'rgba(91,156,246,0.9)';
+			ctx.lineWidth = 2 / this.#cam_z;
+			ctx.stroke();
+		});
+
 		ctx.restore();
 	}
 }