Datepicker: Abstract mouseover logic to avoid explicit event trigger

The reliance on `.mouseover()` caused an issue in some circumstances
(see #5816). The removal of `.mouseover()` broke keyboard navigation
(see #10319).

Fixes #10319
Closes gh-1290

ui/datepicker.js

@@ -799,12 +799,16 @@ $.extend(Datepicker.prototype, {
 		datepicker_instActive = inst; // for delegate hover events
 		inst.dpDiv.empty().append(this._generateHTML(inst));
 		this._attachHandlers(inst);
-		inst.dpDiv.find("." + this._dayOverClass + " a");
 
 		var origyearshtml,
 			numMonths = this._getNumberOfMonths(inst),
 			cols = numMonths[1],
-			width = 17;
+			width = 17,
+			activeCell = inst.dpDiv.find( "." + this._dayOverClass + " a" );
+
+		if ( activeCell.length > 0 ) {
+			datepicker_handleMouseover.apply( activeCell.get( 0 ) );
+		}
 
 		inst.dpDiv.removeClass("ui-datepicker-multi-2 ui-datepicker-multi-3 ui-datepicker-multi-4").width("");
 		if (cols > 1) {
@@ -2000,18 +2004,20 @@ function datepicker_bindHover(dpDiv) {
 				$(this).removeClass("ui-datepicker-next-hover");
 			}
 		})
-		.delegate(selector, "mouseover", function(){
-			if (!$.datepicker._isDisabledDatepicker( datepicker_instActive.inline ? dpDiv.parent()[0] : datepicker_instActive.input[0])) {
-				$(this).parents(".ui-datepicker-calendar").find("a").removeClass("ui-state-hover");
-				$(this).addClass("ui-state-hover");
-				if (this.className.indexOf("ui-datepicker-prev") !== -1) {
-					$(this).addClass("ui-datepicker-prev-hover");
-				}
-				if (this.className.indexOf("ui-datepicker-next") !== -1) {
-					$(this).addClass("ui-datepicker-next-hover");
-				}
-			}
-		});
+		.delegate( selector, "mouseover", datepicker_handleMouseover );
+}
+
+function datepicker_handleMouseover() {
+	if (!$.datepicker._isDisabledDatepicker( datepicker_instActive.inline? datepicker_instActive.dpDiv.parent()[0] : datepicker_instActive.input[0])) {
+		$(this).parents(".ui-datepicker-calendar").find("a").removeClass("ui-state-hover");
+		$(this).addClass("ui-state-hover");
+		if (this.className.indexOf("ui-datepicker-prev") !== -1) {
+			$(this).addClass("ui-datepicker-prev-hover");
+		}
+		if (this.className.indexOf("ui-datepicker-next") !== -1) {
+			$(this).addClass("ui-datepicker-next-hover");
+		}
+	}
 }
 
 /* jQuery extend now ignores nulls! */