Datepicker: Hide the UI on destroy

When the datepicker UI is shown and then destroyed programmatically:
```js
$( "#datepicker" ).datepicker( "destroy" );
```
hide the datepicker UI without the need for an explicit user action.
Previously, in 1.12 the UI would not disappear immediately but only after the
first `mousedown`. In later 1.13 versions, the UI would not disappear at all.

Fixes gh-2178
Closes gh-2268

tests/unit/datepicker/methods.js

@@ -11,7 +11,7 @@ var beforeAfterEach = testHelper.beforeAfterEach;
 QUnit.module( "datepicker: methods", beforeAfterEach()  );
 
 QUnit.test( "destroy", function( assert ) {
-	assert.expect( 35 );
+	assert.expect( 39 );
 	var inl,
 		inp = testHelper.init( "#inp" ),
 		dp = $( "#ui-datepicker-div" );
@@ -21,6 +21,15 @@ QUnit.test( "destroy", function( assert ) {
 	assert.equal( dp.css( "display" ), "block", "Datepicker - visible" );
 	inp.datepicker( "hide" ).datepicker( "destroy" );
 	assert.ok( $.datepicker._curInst == null, "Datepicker - destroyed and cleared reference" );
+	assert.equal( dp.css( "display" ), "none", "Datepicker - absent" );
+
+	// Destroy without manual hiding (ensure datepicker is hidden after calling destroy)
+	inp = testHelper.init( "#inp" );
+	inp.datepicker( "show" );
+	assert.equal( dp.css( "display" ), "block", "Datepicker - visible" );
+	inp.datepicker( "destroy" );
+	assert.ok( $.datepicker._curInst == null, "Datepicker - destroyed and cleared reference" );
+	assert.equal( dp.css( "display" ), "none", "Datepicker - absent" );
 
 	inp = testHelper.init( "#inp" );
 	assert.ok( inp.is( ".hasDatepicker" ), "Default - marker class set" );

ui/widgets/datepicker.js

@@ -435,6 +435,7 @@ $.extend( Datepicker.prototype, {
 			$target.removeClass( this.markerClassName ).empty();
 		}
 
+		$.datepicker._hideDatepicker();
 		if ( datepicker_instActive === inst ) {
 			datepicker_instActive = null;
 			this._curInst = null;