The attached screenshot shows a number of applications that use Client Side Decoration and Header Bars. They are pictured here running on Ubuntu MATE 16.04 daily using the Marco window manager with software compositing enabled. You'll notice that shadows are fully rendered, and the applications that support it, are also resizeable. Pop-overs are also correctly rendered. The CSD support also works without a compositor, naturally there will be no shadows in this case.
To top off this integration the buttons and scrollbars in the GTK2 and GTK3 themes are now much more closely aligned, and from work completed earlier in the 16.04 cycle, Qt4 and Qt5 applications will inherit appearance properties from the GTK widgets. This all helps create a cohesive look and feel for all applications regardless of the toolkit used or design ethos.
You may not be a fan of CSD applications, but there are a good number of them and undoubtedly there will be many more released over the coming years. It would be remiss of the Ubuntu MATE team to not provide first class CSD application support for the upcoming LTS release. With these changes, Ubuntu MATE can finally bridge the gap between traditional and contemporary applications.
Other notable items in this screenshot include the new version of Plank, which now features Docklets, and the Synapse indicator that shows full Synapse integration is finally complete too.