You need to call the search method by ajax too.
Change the index.html and then
1- if Form is valid replace the whole html or the mainContainer( The div that i have added to view).
2- else just replace the partial view.
@{ ViewBag.Title = "App Selection"; }
<div id="mainContainer">
<h2>App Selection</h2>
@Html.Label("Select Type:")
@Html.DropDownListFor(x => x.Platform, Model.Options)
<div id="AppForm"></div> // This is where the Partial View goes
</div>
Remove the form tag from your partial view just call an ajax call method for searching. May be easiest way to handle this problem is using MVC unobtrusive ajax.