The space is called the address bar.
The Space Bar happened in 1997.
Hold Down the space bar while powering up the terminal, then you can setup the address etc on it.
The space bar disappears when a space would be an illegal character. Probably in Safari you were typing in the URL box (perhaps you didn't realize that is where you cursor was) and a space is not a valid URL character. If you had been in the Safari search box it would be a legal character. So be aware of where you cursor is the next time this happens and I'm sure the ipad is trying to protect you from typing an illegal URL.
By putting the bar line
Menu Bar is where you can change browser settings like font, page etc. Address bar is where you can type a web address.
Right click on the address bar.
It means it appears in the address bar
Click on the address bar and then press F11.
Move your mouse up to the tabs. Right click on an empty space and then click navigation toolbar.
They are both keys on the keyboard but a space bar just moves a small space compared to a tab bar. Usually a tab bar is used for indenting, ana space bar is used for separation of words.
You can use either thumb to touch the space bar.