Avoid re-rendering children on click event

[dinkinflickaa]

I noticed that clicking on a DnD child node can be costly because it triggers all child nodes to rerender twice. This issue has been reported before. See #1379

This happens when DndContext updates the activeSensor and activatorEvent states on a mouse down event. These state changes cause rerenders in all consumers of DndContext. Consumers of useSortable re-render too because SortableContext depends on DndContext.

If the user is just clicking or if activation constraints aren’t met, DndContext resets these states to null, causing another render cycle.

Here’s what it looks like in React Profiler for a click event, where you'll see back-to-back commit cycles causing unnecessary rerenders for every child:

I don't see why we need to update state unless the user is actually dragging. This PR proposes delaying the activeSensor and activatorEvent state updates until the drag event starts and constraints are met. This reduces unnecessary re-renders, saving two render cycles for every child.

Here’s what the React Profiler looks like with this change:

From Perf trace, you'll notice that mouse down and mouse up events aren't doing much work
Before:

After:

Cypress tests are passing -

[changeset-bot]

🦋 Changeset detected

Latest commit: b3de0f7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@dnd-kit/core	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[alloy]

@dinkinflickaa Can you please run the e2e tests locally and show that they pass? Also, can the e2e tests be amended any way to protect against this regressing in the future? (I assume it would require more tooling, such as integrating React Render Tracker?)

[dinkinflickaa]

@alloy Thanks for pointing out tests. This helped me uncover a bug which I have fixed now. Added a screenshot of passing tests to PR description.