t-hamano · Jul 21, 2026 · Jul 20, 2026 · Jul 21, 2026
diff --git a/lib/media/load.php b/lib/media/load.php
@@ -199,17 +199,22 @@ function gutenberg_media_processing_filter_rest_index( WP_REST_Response $respons
 add_filter( 'rest_index', 'gutenberg_media_processing_filter_rest_index' );
 
 /**
- * Sets a global JS variable to indicate that HEIC canvas-based upload support is available.
+ * Sets a global JS variable to indicate that client-side media processing is enabled.
  *
- * This flag is set whenever the media processing feature is enabled,
- * regardless of whether the browser supports full VIPS-based processing.
- * Browsers like Safari can use createImageBitmap() to decode HEIC images
- * and convert them to JPEG for server-side sub-size generation.
+ * The flag gates both processing modes: the full VIPS/WASM pipeline (browsers
+ * that pass feature detection) and the HEIC canvas fallback used by browsers
+ * such as Safari that can decode HEIC via createImageBitmap() but lack
+ * SharedArrayBuffer support. The browser-capability check happens client-side.
  */
-function gutenberg_set_heic_upload_support_flag() {
-	wp_add_inline_script( 'wp-block-editor', 'window.__heicUploadSupport = true', 'before' );
+function gutenberg_set_client_side_media_processing_flag() {
+	// Re-check the filter at action time, since other plugins (loaded after Gutenberg)
+	// may have added a filter to disable client-side media processing.
+	if ( ! gutenberg_is_client_side_media_processing_enabled() ) {
+		return;
+	}
+	wp_add_inline_script( 'wp-block-editor', 'window.__clientSideMediaProcessing = true', 'before' );
 }
-add_action( 'admin_init', 'gutenberg_set_heic_upload_support_flag' );
+add_action( 'admin_init', 'gutenberg_set_client_side_media_processing_flag' );
 
 /**
  * Deletes the source-format companion file when its attachment is deleted.
@@ -258,17 +263,6 @@ function gutenberg_delete_heic_companion_file( int $post_id ): bool {
 // Everything below requires cross-origin isolation (Document-Isolation-Policy)
 // and SharedArrayBuffer support, which is only available in Chromium 137+.
 
-/**
- * Sets a global JS variable to indicate that client-side media processing is enabled.
- */
-function gutenberg_set_client_side_media_processing_flag() {
-	if ( ! gutenberg_is_client_side_media_processing_enabled() ) {
-		return;
-	}
-	wp_add_inline_script( 'wp-block-editor', 'window.__clientSideMediaProcessing = true', 'before' );
-}
-add_action( 'admin_init', 'gutenberg_set_client_side_media_processing_flag' );
-
 /**
  * Filters the list of rewrite rules formatted for output to an .htaccess file.
  *

diff --git a/packages/block-editor/CHANGELOG.md b/packages/block-editor/CHANGELOG.md
@@ -6,6 +6,10 @@
 
 -   Inspector controls in the standard block-supports panels (Typography, Dimensions, Border, Color, Background, Filters) now reflect the value a block inherits from Global Styles when no local override is set. Inherited controls show that value at rest (as a placeholder, preselected option, or resolved value) and mark the label with a dotted underline; setting a local override reveals a reset affordance that clears the override back to the inherited value ([#77894](https://github.com/WordPress/gutenberg/pull/77894)).
 
+### Bug Fixes
+
+-   Gate the HEIC canvas conversion fallback on `window.__clientSideMediaProcessing` instead of the redundant `window.__heicUploadSupport` flag, fixing client-side HEIC conversion in Safari on core WordPress installs ([#80452](https://github.com/WordPress/gutenberg/pull/80452)).
+
 ## 16.0.0 (2026-07-14)
 
 ### Internal

diff --git a/packages/block-editor/src/components/provider/index.js b/packages/block-editor/src/components/provider/index.js
@@ -102,7 +102,7 @@ function shouldEnableClientSideMediaProcessing() {
  *
  * Returns true when:
  * 1. Full client-side processing is NOT available (otherwise it handles HEIC already)
- * 2. The server has set the __heicUploadSupport flag
+ * 2. The server has set the __clientSideMediaProcessing flag
  * 3. The browser supports createImageBitmap + OffscreenCanvas (e.g. Safari)
  *
  * @return {boolean} Whether HEIC-only canvas processing should be enabled.
@@ -118,7 +118,7 @@ function shouldEnableHeicCanvasProcessing() {
 		return false;
 	}
 
-	if ( ! window.__heicUploadSupport ) {
+	if ( ! window.__clientSideMediaProcessing ) {
 		isHeicCanvasEnabledCache = false;
 		return false;
 	}

diff --git a/packages/block-library/CHANGELOG.md b/packages/block-library/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Internal
+
+-   Image: Check only `window.__clientSideMediaProcessing` for sideloading status, following the removal of the redundant `window.__heicUploadSupport` flag ([#80452](https://github.com/WordPress/gutenberg/pull/80452)).
+
 ## 10.2.0 (2026-07-14)
 
 ### Enhancements

diff --git a/packages/block-library/src/image/edit.js b/packages/block-library/src/image/edit.js
@@ -353,11 +353,7 @@ export function ImageEdit( {
 
 	const isSideloading = useSelect(
 		( select ) => {
-			if (
-				( ! window.__clientSideMediaProcessing &&
-					! window.__heicUploadSupport ) ||
-				! id
-			) {
+			if ( ! window.__clientSideMediaProcessing || ! id ) {
 				return false;
 			}
 			return select( uploadStore ).isUploadingById( id );

diff --git a/packages/e2e-tests/plugins/disable-cross-origin-isolation.php b/packages/e2e-tests/plugins/disable-cross-origin-isolation.php
@@ -0,0 +1,18 @@
+<?php
+/**
+ * Plugin Name: Gutenberg Test Plugin: Disable Cross-Origin Isolation
+ * Plugin URI: https://github.com/WordPress/gutenberg
+ * Author: Gutenberg Team
+ *
+ * @package gutenberg-test-disable-cross-origin-isolation
+ */
+
+/*
+ * Skip the Document-Isolation-Policy header while leaving client-side media
+ * processing enabled. Without cross-origin isolation, SharedArrayBuffer is
+ * unavailable, so the full VIPS/WASM pipeline fails feature detection and the
+ * editor falls back to the HEIC canvas conversion mode — mirroring Safari,
+ * where full client-side processing is unsupported but HEIC files are still
+ * converted to JPEG via createImageBitmap + OffscreenCanvas before upload.
+ */
+add_filter( 'gutenberg_use_document_isolation_policy', '__return_false' );
diff --git a/packages/media-utils/CHANGELOG.md b/packages/media-utils/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Internal
+
+-   Remove the `window.__heicUploadSupport` type declaration, following the removal of the redundant flag ([#80452](https://github.com/WordPress/gutenberg/pull/80452)).
+
 ## 5.51.0 (2026-07-14)
 
 ### Enhancements

diff --git a/packages/media-utils/src/utils/upload-media.ts b/packages/media-utils/src/utils/upload-media.ts
@@ -22,7 +22,6 @@ import { UploadError } from './upload-error';
 declare global {
 	interface Window {
 		__clientSideMediaProcessing?: boolean;
-		__heicUploadSupport?: boolean;
 	}
 }
 

diff --git a/test/e2e/specs/editor/various/heic-upload-progress-snackbar.spec.js b/test/e2e/specs/editor/various/heic-upload-progress-snackbar.spec.js
@@ -11,13 +11,15 @@ const { test, expect } = require( '@wordpress/e2e-test-utils-playwright' );
 
 test.describe( 'Upload progress snackbar (HEIC-only canvas mode) (@webkit)', () => {
 	test.beforeAll( async ( { requestUtils } ) => {
-		// Disable full client-side media processing while leaving the
-		// HEIC canvas-conversion mode active (`window.__heicUploadSupport`
-		// is set regardless). This mirrors Safari, where full client-side
-		// processing is unsupported but HEIC files are still converted to
-		// JPEG via createImageBitmap + OffscreenCanvas before upload.
+		// Disable cross-origin isolation while leaving client-side media
+		// processing enabled: without SharedArrayBuffer the full VIPS
+		// pipeline fails feature detection, so the editor falls back to the
+		// HEIC canvas-conversion mode. This mirrors Safari, where full
+		// client-side processing is unsupported but HEIC files are still
+		// converted to JPEG via createImageBitmap + OffscreenCanvas before
+		// upload.
 		await requestUtils.activatePlugin(
-			'gutenberg-test-plugin-disable-client-side-media-processing'
+			'gutenberg-test-plugin-disable-cross-origin-isolation'
 		);
 	} );
 
@@ -31,7 +33,7 @@ test.describe( 'Upload progress snackbar (HEIC-only canvas mode) (@webkit)', ()
 
 	test.afterAll( async ( { requestUtils } ) => {
 		await requestUtils.deactivatePlugin(
-			'gutenberg-test-plugin-disable-client-side-media-processing'
+			'gutenberg-test-plugin-disable-cross-origin-isolation'
 		);
 	} );
 
@@ -59,6 +61,19 @@ test.describe( 'Upload progress snackbar (HEIC-only canvas mode) (@webkit)', ()
 		const imageBlock = editor.canvas.locator(
 			'role=document[name="Block: Image"i]'
 		);
+
+		// The canvas conversion mode sideloads the converted JPEG. A plain
+		// server-side upload never hits the sideload endpoint, so waiting for
+		// it proves the HEIC canvas path actually ran: the server also
+		// content-sniffs the fake HEIC's JPEG bytes and renames the file, so
+		// the src assertion below cannot tell the two paths apart on its own.
+		const sideloadRequest = page.waitForRequest(
+			( request ) =>
+				request.method() === 'POST' &&
+				request.url().includes( 'sideload' ),
+			{ timeout: 60_000 }
+		);
+
 		await imageBlock
 			.locator( 'data-testid=form-file-upload-input' )
 			.setInputFiles( {
@@ -67,6 +82,8 @@ test.describe( 'Upload progress snackbar (HEIC-only canvas mode) (@webkit)', ()
 				buffer,
 			} );
 
+		await sideloadRequest;
+
 		const snackbarList = page.locator( '.components-snackbar-list' );
 
 		// A single file must use the single-file message throughout. The