Dialog: Add aria-modal support

Reflect the `modal` dialog option into the `aria-modal` attribute -
when `modal` is `true`, set `aria-modal` to `"true"`.

This helps some accessibility tools like VoiceOver with their rotor
functionality as it reduces the number of elements presented.

Fixes gh-2246
Closes gh-2257

Co-authored-by: Michał Gołębiowski-Owczarek <m.goleb@gmail.com>

tests/unit/dialog/core.js

@@ -84,6 +84,39 @@ QUnit.test( "ARIA", function( assert ) {
 	element.remove();
 } );
 
+QUnit.test( "aria-modal", function( assert ) {
+	assert.expect( 9 );
+
+	var element, wrapper;
+
+	element = $( "<div>" ).dialog( { modal: true } );
+	wrapper = element.dialog( "widget" );
+	assert.equal( wrapper.attr( "aria-modal" ), "true", "modal option set to true, aria-modal attribute added" );
+	element.dialog( "option", "modal", false );
+	assert.equal( wrapper.attr( "aria-modal" ), undefined, "modal option set to false, aria-modal attribute not added" );
+	element.dialog( "option", "modal", true );
+	assert.equal( wrapper.attr( "aria-modal" ), "true", "modal option set to true, aria-modal attribute added" );
+	element.remove();
+
+	element = $( "<div>" ).dialog( { modal: false } );
+	wrapper = element.dialog( "widget" );
+	assert.equal( wrapper.attr( "aria-modal" ), undefined, "modal option set to false, aria-modal attribute not added" );
+	element.dialog( "option", "modal", true );
+	assert.equal( wrapper.attr( "aria-modal" ), "true", "modal option set to true, aria-modal attribute added" );
+	element.dialog( "option", "modal", false );
+	assert.equal( wrapper.attr( "aria-modal" ), undefined, "modal option set to false, aria-modal attribute not added" );
+	element.remove();
+
+	element = $( "<div>" ).dialog();
+	wrapper = element.dialog( "widget" );
+	assert.equal( wrapper.attr( "aria-modal" ), undefined, "modal option not set, aria-modal attribute not added" );
+	element.dialog( "option", "modal", true );
+	assert.equal( wrapper.attr( "aria-modal" ), "true", "modal option set to true, aria-modal attribute added" );
+	element.dialog( "option", "modal", false );
+	assert.equal( wrapper.attr( "aria-modal" ), undefined, "modal option set to false, aria-modal attribute not added" );
+	element.remove();
+} );
+
 QUnit.test( "widget method", function( assert ) {
 	assert.expect( 1 );
 	var dialog = $( "<div>" ).appendTo( "#qunit-fixture" ).dialog();

ui/widgets/dialog.js

@@ -347,7 +347,8 @@ $.widget( "ui.dialog", {
 
 				// Setting tabIndex makes the div focusable
 				tabIndex: -1,
-				role: "dialog"
+				role: "dialog",
+				"aria-modal": this.options.modal ? "true" : null
 			} )
 			.appendTo( this._appendTo() );
 
@@ -762,6 +763,10 @@ $.widget( "ui.dialog", {
 		if ( key === "title" ) {
 			this._title( this.uiDialogTitlebar.find( ".ui-dialog-title" ) );
 		}
+
+		if ( key === "modal" ) {
+			uiDialog.attr( "aria-modal", value ? "true" : null );
+		}
 	},
 
 	_size: function() {