Fix: Media upload from the post edit screen shows an inaccurate number when uploading multiple media files

src/js/media/models/attachments.js

@@ -212,8 +212,6 @@ var Attachments = Backbone.Collection.extend(/** @lends wp.media.model.Attachmen
 		this.observers.push( attachments );
 
 		attachments.on( 'add change remove', this._validateHandler, this );
-		attachments.on( 'add', this._addToTotalAttachments, this );
-		attachments.on( 'remove', this._removeFromTotalAttachments, this );
 		attachments.on( 'reset', this._validateAllHandler, this );
 		this.validateAll( attachments );
 		return this;
@@ -310,6 +308,12 @@ var Attachments = Backbone.Collection.extend(/** @lends wp.media.model.Attachmen
 		this.reset( [], { silent: true } );
 		this.observe( attachments );
 
+		// Bind total attachment count tracking only to the mirrored query
+		// collection, not to additional observed collections (e.g. selection).
+		// See Trac #65053.
+		attachments.on( 'add', this._addToTotalAttachments, this );
+		attachments.on( 'remove', this._removeFromTotalAttachments, this );
+
 		// Used for the search results.
 		this.trigger( 'attachments:received', this );
 		return this;

tests/qunit/index.html

@@ -166,6 +166,7 @@
 		<script src="wp-admin/js/widgets/test-media-image-widget.js"></script>
 		<script src="wp-admin/js/widgets/test-media-gallery-widget.js"></script>
 		<script src="wp-admin/js/widgets/test-media-video-widget.js"></script>
+		<script src="wp-includes/js/media/test-models.js"></script>
 
 		<!-- Customizer templates for sections -->
 		<script type="text/html" id="tmpl-customize-section-default">

tests/qunit/wp-includes/js/media/test-models.js

@@ -0,0 +1,107 @@
+/* globals wp */
+/* jshint qunit: true */
+/* eslint-env qunit */
+
+( function () {
+	'use strict';
+
+	QUnit.module( 'wp.media.model.Attachments' );
+
+	/*
+	 * Trac #65053: opening the media modal via "Set featured image" or
+	 * an Image block reported an incorrect "Showing X of Y" total because
+	 * Attachments#observe() bound the totalAttachments trackers to every
+	 * observed collection, including the Selection. Selection mutations
+	 * therefore mutated the mirrored Query's totalAttachments.
+	 *
+	 * The library mirrors a Query (which owns totalAttachments) AND
+	 * additionally observes a Selection (so a pre-existing featured image
+	 * stays visible in the grid). Only the Query should drive the count.
+	 */
+	QUnit.test(
+		'observing a selection does not corrupt totalAttachments (Trac #65053)',
+		function ( assert ) {
+			var Attachments = wp.media.model.Attachments,
+				Selection = wp.media.model.Selection,
+				Attachment = wp.media.model.Attachment,
+				query,
+				library,
+				selection;
+
+			// Stand-in for a Query collection: the source of truth for totalAttachments.
+			query = new Attachments();
+			query.totalAttachments = 8;
+
+			// The library is what AttachmentsBrowser renders. It mirrors the query.
+			library = new Attachments();
+			library.mirror( query );
+
+			assert.strictEqual(
+				library.getTotalAttachments(),
+				8,
+				'precondition: getTotalAttachments() reflects the mirrored query total'
+			);
+
+			// FeaturedImage / ReplaceImage controllers additionally observe the selection.
+			selection = new Selection( [], { multiple: false } );
+			library.observe( selection );
+
+			// Adding to the selection must not mutate the mirrored query's total.
+			selection.add( new Attachment( { id: 42 } ) );
+			assert.strictEqual(
+				library.getTotalAttachments(),
+				8,
+				'add to observed selection leaves totalAttachments untouched'
+			);
+
+			// Single-select swap: Selection#add internally remove()s prior models
+			// before adding the new one. Both events must be ignored by the count.
+			selection.add( new Attachment( { id: 99 } ) );
+			assert.strictEqual(
+				library.getTotalAttachments(),
+				8,
+				'single-select swap on observed selection leaves totalAttachments untouched'
+			);
+
+			// Removing from the selection must also leave the total alone.
+			selection.remove( selection.models );
+			assert.strictEqual(
+				library.getTotalAttachments(),
+				8,
+				'remove from observed selection leaves totalAttachments untouched'
+			);
+		}
+	);
+
+	QUnit.test(
+		'mirrored query add/remove still drives totalAttachments',
+		function ( assert ) {
+			var Attachments = wp.media.model.Attachments,
+				Attachment = wp.media.model.Attachment,
+				query,
+				library,
+				attachment;
+
+			query = new Attachments();
+			query.totalAttachments = 5;
+
+			library = new Attachments();
+			library.mirror( query );
+
+			attachment = new Attachment( { id: 1 } );
+			query.add( attachment );
+			assert.strictEqual(
+				library.getTotalAttachments(),
+				6,
+				'adding to the mirrored query increments totalAttachments'
+			);
+
+			query.remove( attachment );
+			assert.strictEqual(
+				library.getTotalAttachments(),
+				5,
+				'removing from the mirrored query decrements totalAttachments'
+			);
+		}
+	);
+} )();