Fix rendering issues when both chunking and React's strict mode are enabled

[12joan]

Description
If React's strict mode and the experimental chunking optimisation are both enabled, this caused rendering issues inside chunks that prevented slate-react from synchronising changes to the editor value with the DOM. This PR resolves these issues by adjusting the timing of when the chunk tree's modifiedChunks set is cleared.

Example
Recording of the bug as reported by z2devil on Slack:

When the editor chunk is initialized in React StrictMode, text input inside this chunk stops working and the cursor disappears.

20251210170131.mp4

Context
The chunking optimisation works by iteratively mutating a nested "chunk tree" object as part of useChildren every time the parent element renders. On each iteration, the new children array is "reconciled" with the chunk tree, analogous to how React reconciles the DOM with its virtual DOM. This reconciliation can result in chunks being added, removed or modified.

Since each individual chunk in the chunk tree is mutable, React cannot automatically determine whether the component corresponding to each chunk needs to be re-rendered. For this reason, the chunk tree maintains a set of modifiedChunks, which are chunks that have been modified since the last render. If a given chunk appears in this set, we bypass React's memoisation and re-render the chunk's component.

Prior to this PR, this modifiedChunks set was cleared at the start of each iteration (i.e. at the start of each render), so that the set included only those chunks that were modified as part of the current iteration. However, this caused issues with React's strict mode, in which each component's render function is called twice prior to that component actually being rendered.

This meant that on the first render pass, modifiedChunks was correct but unused. On the second render pass (caused by strict mode), modifiedChunks had already been cleared, and the chunk tree had already been updated. This meant that modifiedChunk was empty when React.memo checked whether each chunk component needed to be re-rendered, resulting in the chunk component failing to update in response to changes to the editor value.

This PR fixes this by only clearing modifiedChunks inside a useEffect, by which time its value has already been used.

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot]

🦋 Changeset detected

Latest commit: 32d56dd

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

This PR includes changesets to release 1 package

Name	Type
slate-react	Patch

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[12joan]

Adds a checkbox to the huge document example to enable strict mode for testing. Due to the nature of strict mode, this only works when running the examples locally.

[12joan]

This file was changed as a result of yarn tsc:examples