My solution but may only be for me. It is design view disappearing but not from a custom control rather my stylesheet.

I've had a similar issue. HTML/source was fine, would run fine but the second I went into Design or Split view, nothing was there.

It was working fine all day and then it "just happened". I'm back to normal now and this is what I found. It all stemmed from my StyleSheet (despite that it was working fine for weeks). I have a "generic" #Wrapper that I have used on every project I've built. I use it for the main DIV (form) on my pages.

Here was my solution (and a theory on how it got jacked up in the first place.)

1. First, make sure all "open" pages are in Source view, including the master page.
2. Add a NEW stylesheet to your project. In the master page, rename your referenced stylesheet to the new (blank) stylesheet's name.
3. Now check each page in Design/Split view and you should see something (ugly since there is no formatting but something). Put everything back into Source view.
4. Next, I copied the first section of the "bad" stylesheet and pasted it in the "new" stylesheet. Repeat Step 3.
5. If you still see your controls in Design/Split view then copy and paste the next section from the "bad" stylesheet.
6. Repeat the process until your Design/Split view goes "bonkers" again. You should now know the offending section. Do not include it in your "new" stylesheet. (Renaming will not work either.)

My theory: While I was debugging, I would tweak the stylesheet to accomodate browser compatability issues and then refresh the page to see the changes. (I've done this in the past without a problem.) I believe that since I was working on the stylesheet while it was debugging, a file (temp or otherwise) became corrupted. Rebooting, and several other internet solutions did not work. This was the only thing that worked for my situation and in all actuality, this was much less time consuming then having to recreate the entire project again.

Hope this helps... Now back to work, finally.