chore: null out current_batch before committing branches (#17809)

Another small tweak extracted from #17805, just to make that diff a bit
more legible.

By passing the `batch` to the branch commit callback, we don't need to
rely on the value of `current_batch` being the same as the batch
currently being processed. That gives us more control over the order of
operations — for example we can null out `current_batch` _before_
committing branches, which is important (at present, if a state change
occurs while those branches are being committed, it will belong to the
current batch, but the resulting effects will happen in the context of a
_new_ batch, which is something we need to avoid for the sake of
#17805).

### Before submitting the PR, please make sure you do the following

- [x] It's really useful if your PR references an issue where it is
discussed ahead of time. In many cases, features are absent for a
reason. For large changes, please create an RFC:
https://github.com/sveltejs/rfcs
- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [ ] Ideally, include a test that fails without this PR but passes with
it.
- [x] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).

### Tests and linting

- [x] Run the tests with `pnpm test` and lint the project with `pnpm
lint`

.changeset/vast-ties-wash.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: null out current_batch before committing branches

packages/svelte/src/internal/client/dom/blocks/branches.js

@@ -68,9 +68,10 @@ export class BranchManager {
 		this.#transition = transition;
 	}
 
-	#commit = () => {
-		var batch = /** @type {Batch} */ (current_batch);
-
+	/**
+	 * @param {Batch} batch
+	 */
+	#commit = (batch) => {
 		// if this batch was made obsolete, bail
 		if (!this.#batches.has(batch)) return;
 
@@ -221,7 +222,7 @@ export class BranchManager {
 				this.anchor = hydrate_node;
 			}
 
-			this.#commit();
+			this.#commit(batch);
 		}
 	}
 }

packages/svelte/src/internal/client/reactivity/batch.js

@@ -87,7 +87,7 @@ export class Batch {
 	/**
 	 * When the batch is committed (and the DOM is updated), we need to remove old branches
 	 * and append new ones by calling the functions added inside (if/each/key/etc) blocks
-	 * @type {Set<() => void>}
+	 * @type {Set<(batch: Batch) => void>}
 	 */
 	#commit_callbacks = new Set();
 
@@ -207,19 +207,19 @@ export class Batch {
 				reset_branch(e, t);
 			}
 		} else {
+			// If sources are written to, then work needs to happen in a separate batch, else prior sources would be mixed with
+			// newly updated sources, which could lead to infinite loops when effects run over and over again.
+			previous_batch = this;
+			current_batch = null;
+
 			// append/remove branches
-			for (const fn of this.#commit_callbacks) fn();
+			for (const fn of this.#commit_callbacks) fn(this);
 			this.#commit_callbacks.clear();
 
 			if (this.#pending === 0) {
 				this.#commit();
 			}
 
-			// If sources are written to, then work needs to happen in a separate batch, else prior sources would be mixed with
-			// newly updated sources, which could lead to infinite loops when effects run over and over again.
-			previous_batch = this;
-			current_batch = null;
-
 			flush_queued_effects(render_effects);
 			flush_queued_effects(effects);
 
@@ -358,6 +358,7 @@ export class Batch {
 		if (batches.size > 1) {
 			this.previous.clear();
 
+			var previous_batch = current_batch;
 			var previous_batch_values = batch_values;
 			var is_earlier = true;
 
@@ -421,7 +422,7 @@ export class Batch {
 				}
 			}
 
-			current_batch = null;
+			current_batch = previous_batch;
 			batch_values = previous_batch_values;
 		}
 
@@ -479,7 +480,7 @@ export class Batch {
 		this.flush();
 	}
 
-	/** @param {() => void} fn */
+	/** @param {(batch: Batch) => void} fn */
 	oncommit(fn) {
 		this.#commit_callbacks.add(fn);
 	}