test(drag-drop): add reorder logic tests for useBlockDragDrop

Adds simulateDragDrop helper and three tests covering the splice/insertAt
index arithmetic in handlePointerUp:
- move-to-end (insertAt path where target > fromIdx)
- move-to-start (insertAt path where target <= fromIdx)
- move-down-by-one (verifies the off-by-one dropTargetIdx - 1 branch)

Fixes @saraholt: "reorder calculation in handlePointerUp is untested"

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

frontend/src/lib/hooks/__tests__/useBlockDragDrop.svelte.test.ts

@@ -16,6 +16,78 @@ function makeBlock(id: string, sortOrder: number): TranscriptionBlockData {
 	};
 }
 
+/**
+ * Builds a DOM list, mocks getBoundingClientRect (60px per wrapper),
+ * drags `dragId` and drops it so dropTargetIdx === targetIdx, then
+ * triggers handlePointerUp. Returns the onReorder spy.
+ */
+function simulateDragDrop(
+	dragId: string,
+	targetIdx: number,
+	blocks: TranscriptionBlockData[]
+): ReturnType<typeof vi.fn> {
+	const onReorder = vi.fn();
+	const dd = createBlockDragDrop({ getSortedBlocks: () => blocks, onReorder });
+
+	// Build DOM
+	const listEl = document.createElement('div');
+	const wrappers = blocks.map(() => {
+		const grip = document.createElement('div');
+		grip.setAttribute('data-drag-handle', '');
+		const wrapper = document.createElement('div');
+		wrapper.setAttribute('data-block-wrapper', '');
+		wrapper.appendChild(grip);
+		listEl.appendChild(wrapper);
+		return { grip, wrapper };
+	});
+	document.body.appendChild(listEl);
+	dd.setListElement(listEl);
+
+	// Mock bounding rects: each wrapper is 60px tall starting at y=0
+	wrappers.forEach(({ wrapper }, i) => {
+		vi.spyOn(wrapper, 'getBoundingClientRect').mockReturnValue({
+			top: i * 60,
+			height: 60,
+			bottom: (i + 1) * 60,
+			left: 0,
+			right: 100,
+			width: 100,
+			x: 0,
+			y: i * 60,
+			toJSON: () => ({})
+		} as DOMRect);
+	});
+
+	const dragIdx = blocks.findIndex((b) => b.id === dragId);
+	const { grip, wrapper: dragWrapper } = wrappers[dragIdx];
+	dragWrapper.setPointerCapture = vi.fn();
+
+	// Start drag
+	const downEvent = new PointerEvent('pointerdown', { clientY: dragIdx * 60, cancelable: true });
+	Object.defineProperty(downEvent, 'target', { value: grip });
+	dd.handleGripDown(downEvent as PointerEvent, dragId);
+
+	// Move pointer to achieve the desired targetIdx
+	// midpoint of wrapper[i] = i*60 + 30
+	// clientY just before midpoint[i] → target = i
+	// clientY past last midpoint → target = wrappers.length
+	let clientY: number;
+	if (targetIdx <= 0) {
+		clientY = 5; // before first midpoint (30)
+	} else if (targetIdx >= wrappers.length) {
+		clientY = wrappers.length * 60 + 10; // past all midpoints
+	} else {
+		clientY = targetIdx * 60 + 5; // just past top of wrapper[targetIdx], before its midpoint
+	}
+
+	const moveEvent = new PointerEvent('pointermove', { clientY });
+	dd.handlePointerMove(moveEvent as PointerEvent);
+	dd.handlePointerUp();
+
+	document.body.removeChild(listEl);
+	return onReorder;
+}
+
 describe('createBlockDragDrop', () => {
 	it('initial state — no drag in progress', () => {
 		const dd = createBlockDragDrop({ getSortedBlocks: () => [], onReorder: vi.fn() });
@@ -55,8 +127,6 @@ describe('createBlockDragDrop', () => {
 		const blocks = [makeBlock('b1', 0), makeBlock('b2', 1)];
 		const dd = createBlockDragDrop({ getSortedBlocks: () => blocks, onReorder });
 
-		// Simulate a drag start without going through handleGripDown internals
-		// by checking that handlePointerUp without a drop target is a no-op for reorder
 		const grip = document.createElement('div');
 		grip.setAttribute('data-drag-handle', '');
 		const wrapper = document.createElement('div');
@@ -76,4 +146,23 @@ describe('createBlockDragDrop', () => {
 
 		document.body.removeChild(wrapper);
 	});
+
+	it('reorder: moves block from index 0 to end', () => {
+		const blocks = [makeBlock('b1', 0), makeBlock('b2', 1), makeBlock('b3', 2)];
+		const onReorder = simulateDragDrop('b1', 3, blocks);
+		expect(onReorder).toHaveBeenCalledWith(['b2', 'b3', 'b1']);
+	});
+
+	it('reorder: moves block from end to index 0', () => {
+		const blocks = [makeBlock('b1', 0), makeBlock('b2', 1), makeBlock('b3', 2)];
+		const onReorder = simulateDragDrop('b3', 0, blocks);
+		expect(onReorder).toHaveBeenCalledWith(['b3', 'b1', 'b2']);
+	});
+
+	it('reorder: moves block down by one position (tests insertAt = dropTargetIdx - 1)', () => {
+		const blocks = [makeBlock('b1', 0), makeBlock('b2', 1), makeBlock('b3', 2)];
+		// dragId=b1 (idx=0), targetIdx=2 → insertAt = 2-1 = 1 → [b2, b1, b3]
+		const onReorder = simulateDragDrop('b1', 2, blocks);
+		expect(onReorder).toHaveBeenCalledWith(['b2', 'b1', 'b3']);
+	});
 });