Let's be clear, I don't really like WebForms. Which is why I do not wish to spend hours debugging a legacy code base that ran perfectly well on one server (IIS7) and a new one (IIS8).
I heard there is this new ClientIDMode, which makes forms and other controls change their IDs or not to something sensible, but I didn't touch it. It seems like the default value has changed (ie is not backwards compatible)?
I had a form in a master page that looked like <form id=SiteForm runat=server
This automatically changed its ID to aspnetForm in the rendered output.
Sure it was a bit weird, but I have a lot of code in this app that relies on it. I could simply change the ID in the server page to <form id=SiteForm runat=aspnetForm and that might fix it. But maybe there's some code in this old project that uses SiteForm on the server side, so I can search for that.
But there could be other controls lurking around which had their IDs changed too. So maybe there is something I can add to the web config to tu the behaviour back to what it was before? What was the default before?