Wasted screen space due to multiple horizontal bars at the top when an application is open in GNOME desktop

Just moved from Unity to GNOME with the update to Ubuntu 17.10. There is a HUGE amount of wasted space at the top of the screen, when an application is open.

Evolution and Firefox both have four "bars" with 90% of each empty. Unity, to its credit was able to blend / mix these horizontal bars so there was only one / two.

Four is way too much. If this is GNOME I may carry on with Unity for the next six months.

Can anybody suggest a tweak/change/fix for this?

You may try one of following GNOME Shell extensions to get rid of the title-bar of a maximised window:

• No Title Bar (an actively maintained fork of Pixel Saver with customisation options)

  No Title Bar removes the title bar, moves the window title and buttons to the top panel.

• Pixel Saver

  Pixel Saver is designed to save pixel by fusing activity bar and title bar in a natural way.

  _{Note that Pixel Saver is not actively maintained anymore, therefore has a lot of unresolved issues like this one and doesn't offer any customisation options.}

Other similar extensions (including forks):

• No Title Bar - Forked
• GTK Title Bar
• Pixel Saver fork

You can even go further and hide the top bar following this answer by solsTiCe from this Q&A: Auto hiding top panel in 17.10 GNOME 3.26.1.

Also, you may move the menu bar from an application window to the top-bar ("Activities bar") following this Q&A: Ubuntu 17.10 windows' menu in panel.

I prefer the Unite GNOME Shell Extension. It manages to integrate far more application windows into the top bar than No Title Bar or Pixel Saver does.