Doesn't seem to happen in a real IE 7, so I wouldn't worry about it. You can put a meta tag on the page to make relatively sure that IE will not use compatibility mode:
Use it as the first meta tag in the head of the page.
<meta http-equiv="X-UA-Compatible" content="IE=Edge" />
Note: This will not prevent someone from switching to compatibility mode, and if they have their browser already in compatibility mode, this might not bring them out of it. But in most cases it (coupled with a standards invoking DOCTYPE like you already have) will force the IE browser to use its most recent rendering engine and not to invoke compatibility mode.