rename run_transitions to should_intro, add explanatory jsdoc

Author: Rich-Harris

packages/svelte/src/internal/client/dom/blocks/svelte-element.js

@@ -10,7 +10,7 @@ import {
 import { remove } from '../reconciler.js';
 import { current_block } from '../../runtime.js';
 import { is_array } from '../../utils.js';
-import { set_run_transitions } from '../../render.js';
+import { set_should_intro } from '../../render.js';
 import { current_each_item_block, set_current_each_item_block } from './each.js';
 
 /**
@@ -104,7 +104,7 @@ export function element(anchor, get_tag, is_svg, render_fn) {
 			} else {
 				// tag is changing — destroy immediately, render contents without intro transitions
 				destroy_effect(effect);
-				set_run_transitions(false);
+				set_should_intro(false);
 			}
 		}
 
@@ -145,7 +145,7 @@ export function element(anchor, get_tag, is_svg, render_fn) {
 
 		tag = next_tag;
 		if (tag) current_tag = tag;
-		set_run_transitions(true);
+		set_should_intro(true);
 
 		set_current_each_item_block(previous_each_item_block);
 	}, block);

packages/svelte/src/internal/client/dom/elements/transitions.js

@@ -3,7 +3,7 @@ import { user_effect } from '../../reactivity/effects.js';
 import { current_effect, untrack } from '../../runtime.js';
 import { raf } from '../../timing.js';
 import { loop } from '../../loop.js';
-import { run_transitions } from '../../render.js';
+import { should_intro } from '../../render.js';
 import { is_function } from '../../utils.js';
 import { current_each_item_block } from '../blocks/each.js';
 import { TRANSITION_GLOBAL, TRANSITION_IN, TRANSITION_OUT } from '../../../../constants.js';
@@ -208,7 +208,7 @@ export function transition(flags, element, get_fn, get_params) {
 	// if this is a local transition, we only want to run it if the parent (block) effect's
 	// parent (branch) effect is where the state change happened. we can determine that by
 	// looking at whether the branch effect is currently initializing
-	if (is_intro && run_transitions) {
+	if (is_intro && should_intro) {
 		var parent = /** @type {import('#client').Effect} */ (effect.parent);
 
 		if (is_global || (parent.f & EFFECT_RAN) !== 0) {

packages/svelte/src/internal/client/render.js

@@ -20,11 +20,16 @@ export const all_registered_events = new Set();
 /** @type {Set<(events: Array<string>) => void>} */
 export const root_event_handles = new Set();
 
-export let run_transitions = true;
+/**
+ * This is normally true — block effects should run their intro transitions —
+ * but is false during hydration and mounting (unless `options.intro` is `true`)
+ * and when creating the children of a `<svelte:element>` that just changed tag
+ */
+export let should_intro = true;
 
 /** @param {boolean} value */
-export function set_run_transitions(value) {
-	run_transitions = value;
+export function set_should_intro(value) {
+	should_intro = value;
 }
 
 /**
@@ -204,7 +209,7 @@ function _mount(Component, options) {
 	const registered_events = new Set();
 	const container = options.target;
 
-	run_transitions = options.intro ?? false;
+	should_intro = options.intro ?? false;
 
 	/** @type {import('#client').RootBlock} */
 	const block = {
@@ -250,7 +255,7 @@ function _mount(Component, options) {
 	const bound_event_listener = handle_event_propagation.bind(null, container);
 	const bound_document_event_listener = handle_event_propagation.bind(null, document);
 
-	run_transitions = true;
+	should_intro = true;
 
 	/** @param {Array<string>} events */
 	const event_handle = (events) => {