I've noticed this too. Sometimes the Start Page window is minimized more than it should be. Then, when I maximize the window, the upper title border disappears.
My workaround has been to click onto that teeny little x --it's extremely tiny-- which is on the right edge of the upper tab bar. This closes Opera. Then, when I open the Opera again, I get my upper title border back. Then I need to take my cursor and very, very, slowly push the window to the left so that I can stretch it to fill the entire screen. However if I don't do this extremely slowly, it won't catch and I won't be able to push the window to the left in order to fill the screen.
This fixes the problem for me in Xubuntu 16.04.
@gbwinter & @jfgorski
Who knows? This might help both you guys. But, that is only if the upper title border is hiding and not completely gone. In my case the upper border hides at times, but is never really gone. That teeny x, on the upper tab bar, is the key. I need to x out, and then reopen the page, and very slowly stretch and extend the page to fill the screen.