Design Manager Bug – SharePoint 2013 --add an app getting loading screen in sharepoint 2013

this issue will come when ever you will migrate share point 2010 to 2013 or if we create minimal master page or by creating new master page ---loading screen will appear/something went wrong 



solution for this is
Make sure you have all placeholders as below:

1. DeltaPlaceHolderLeftNavBar
2. PlaceHolderLeftNavBar
3. DeltaPlaceHolderPageTitleInTitleArea
4.  PlaceHolderPageTitleInTitleArea 
Also each of them should not be set to Visible=False. 

The code for the content placeholders are:

<SharePoint:AjaxDelta id="DeltaPlaceHolderLeftNavBar" BlockElement="true" runat="server">
<asp:ContentPlaceHolder id="PlaceHolderLeftNavBar" runat="server">
</asp:ContentPlaceHolder>
</SharePoint:AjaxDelta
<SharePoint:AjaxDelta id="DeltaPlaceHolderPageTitleInTitleArea" runat="server">
<asp:ContentPlaceHolder id="PlaceHolderPageTitleInTitleArea" runat="server">
</asp:ContentPlaceHolder><asp:ContentPlaceHolder id="PlaceHolderTitleBreadcrumb" runat="server" Visible="false"></asp:ContentPlaceHolder>
</SharePoint:AjaxDelta>

just copy above code to your html master page