How to resolve the "Loading" screen issue that constantly shows while using "Add an App" in sharepoint2013 (especially if you have built your own custom master page).

STEP 1) Ensure you add "DeltaPlaceHolderPageTitleInTitleArea" and "DeltaPlaceHolderLeftNavBar" to the tag below in Master page
<!--SPM:<SharePoint:SPClientIDGenerator runat="server" ServerControlID="DeltaPlaceHolderMain;DeltaPlaceHolderPageTitleInTitleArea;DeltaPlaceHolderUtilityContent;DeltaPlaceHolderLeftNavBar"/>-->


STEP 2) Add a Div hidden snippet just above the footer or at bottom of master page before the </body> end tag, with the following controls (cross check whether the following controls are not already present on the Master page - HINT: Find by the id name - if they exist dont re-add them)
   
<div style="display:none;">
 <!-- remove start and end tags below for DeltaPlaceHolderLeftNavBar and PlaceHolderLeftNavBar if already exists in master page -->
 <!--SPM:<SharePoint:AjaxDelta id="DeltaPlaceHolderLeftNavBar" BlockElement="true" CssClass="ms-core-navigation" role="navigation" runat="server">-->
 <!--SPM:<asp:ContentPlaceHolder id="PlaceHolderLeftNavBar" runat="server">-->
 <!--SPM:</asp:ContentPlaceHolder>-->
 <!--SPM:</SharePoint:AjaxDelta>-->
 <!-- remove start and end tags below for DeltaPlaceHolderPageTitleInTitleArea and PlaceHolderPageTitleInTitleArea if already exists in master page -->
 <!--SPM:<SharePoint:AjaxDelta id="DeltaPlaceHolderPageTitleInTitleArea" runat="server">-->
 <!--SPM:<asp:ContentPlaceHolder id="PlaceHolderPageTitleInTitleArea" runat="server">-->
 <!--SPM:</asp:ContentPlaceHolder>-->
 <!--SPM:</SharePoint:AjaxDelta>-->
</div>