feat(vue-nodes): snap link preview; connect on drop (#5780)

## Summary

Snap link preview to the nearest compatible slot while dragging in Vue
Nodes mode, and complete the connection on drop using the snapped
target. Mirrors LiteGraph’s first-compatible-slot logic for node-level
snapping and reuses the computed candidate for performance.

## Changes

- Snap preview end to compatible slot
  - slot under cursor via `data-slot-key` fast-path
  - node under cursor via `findInputByType` / `findOutputByType`
- Render path
- `slotLinkPreviewRenderer.ts` now renders to
`state.candidate.layout.position`
- Complete on drop
  - Prefer `state.candidate` (no re-hit-testing)
  - Fallbacks: DOM slot → node first-compatible → reroute
  - Disconnects moving input link when dropped on canvas

## Review Focus

- UX feel of snapping and drop completion (both directions)
- Performance on large graphs (mousemove path is O(1) with dataset +
single validation)
- Edge cases: reroutes, moving existing links, collapsed nodes

## Screenshots (if applicable)


https://github.com/user-attachments/assets/fbed0ae2-2231-473b-a05a-9aaf68e3f820

https://github.com/Comfy-Org/ComfyUI_frontend/pull/5780 (snapping) <--
https://github.com/Comfy-Org/ComfyUI_frontend/pull/5898 (drop on canvas
+ linkconnectoradapter refactor) <--
https://github.com/Comfy-Org/ComfyUI_frontend/pull/5903 (fix reroute
snapping)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5780-feat-vue-nodes-snap-link-preview-connect-on-drop-27a6d73d365081d89c8cf570e2049c89)
by [Unito](https://www.unito.io)

---------

Co-authored-by: github-actions <github-actions@github.com>

browser_tests/fixtures/VueNodeHelpers.ts

@@ -13,6 +13,13 @@ export class VueNodeHelpers {
     return this.page.locator('[data-node-id]')
   }
 
+  /**
+   * Get locator for a Vue node by its NodeId
+   */
+  getNodeLocator(nodeId: string): Locator {
+    return this.page.locator(`[data-node-id="${nodeId}"]`)
+  }
+
   /**
    * Get locator for selected Vue node components (using visual selection indicators)
    */

browser_tests/tests/vueNodes/interactions/links/linkInteraction.spec.ts

@@ -693,4 +693,99 @@ test.describe('Vue Node Link Interaction', () => {
       if (shiftHeld) await comfyPage.page.keyboard.up('Shift').catch(() => {})
     }
   })
+
+  test('should snap to node center while dragging and link on drop', async ({
+    comfyPage,
+    comfyMouse
+  }) => {
+    const clipNode = (await comfyPage.getNodeRefsByType('CLIPTextEncode'))[0]
+    const samplerNode = (await comfyPage.getNodeRefsByType('KSampler'))[0]
+    expect(clipNode && samplerNode).toBeTruthy()
+
+    // Start drag from CLIP output[0]
+    const clipOutputCenter = await getSlotCenter(
+      comfyPage.page,
+      clipNode.id,
+      0,
+      false
+    )
+
+    // Drag to the visual center of the KSampler Vue node (not a slot)
+    const samplerVue = comfyPage.vueNodes.getNodeLocator(String(samplerNode.id))
+    await expect(samplerVue).toBeVisible()
+    const samplerCenter = await getCenter(samplerVue)
+
+    await comfyMouse.move(clipOutputCenter)
+    await comfyMouse.drag(samplerCenter)
+
+    // During drag, the preview should snap/highlight a compatible input on KSampler
+    await expect(comfyPage.canvas).toHaveScreenshot('vue-node-snap-to-node.png')
+
+    // Drop to create the link
+    await comfyMouse.drop()
+    await comfyPage.nextFrame()
+
+    // Validate a link was created to one of KSampler's compatible inputs (1 or 2)
+    const linkOnInput1 = await getInputLinkDetails(
+      comfyPage.page,
+      samplerNode.id,
+      1
+    )
+    const linkOnInput2 = await getInputLinkDetails(
+      comfyPage.page,
+      samplerNode.id,
+      2
+    )
+
+    const linked = linkOnInput1 ?? linkOnInput2
+    expect(linked).not.toBeNull()
+    expect(linked?.originId).toBe(clipNode.id)
+    expect(linked?.targetId).toBe(samplerNode.id)
+  })
+
+  test('should snap to a specific compatible slot when targeting it', async ({
+    comfyPage,
+    comfyMouse
+  }) => {
+    const clipNode = (await comfyPage.getNodeRefsByType('CLIPTextEncode'))[0]
+    const samplerNode = (await comfyPage.getNodeRefsByType('KSampler'))[0]
+    expect(clipNode && samplerNode).toBeTruthy()
+
+    // Drag from CLIP output[0] to KSampler input[2] (third slot) which is the
+    // second compatible input for CLIP
+    const clipOutputCenter = await getSlotCenter(
+      comfyPage.page,
+      clipNode.id,
+      0,
+      false
+    )
+    const samplerInput3Center = await getSlotCenter(
+      comfyPage.page,
+      samplerNode.id,
+      2,
+      true
+    )
+
+    await comfyMouse.move(clipOutputCenter)
+    await comfyMouse.drag(samplerInput3Center)
+
+    // Expect the preview to show snapping to the targeted slot
+    await expect(comfyPage.canvas).toHaveScreenshot('vue-node-snap-to-slot.png')
+
+    // Finish the connection
+    await comfyMouse.drop()
+    await comfyPage.nextFrame()
+
+    const linkDetails = await getInputLinkDetails(
+      comfyPage.page,
+      samplerNode.id,
+      2
+    )
+    expect(linkDetails).not.toBeNull()
+    expect(linkDetails).toMatchObject({
+      originId: clipNode.id,
+      targetId: samplerNode.id,
+      targetSlot: 2
+    })
+  })
 })