when I (try to) use a native titlebar I won't get any title bar at all in a KDE Plasma environment. It's no issue when running Gnome.
I tried this on Fedora 23/64-bit with KDE Plasma 5.4.3, Qt 5.5.1. JUCE is from git commit 6f7f97607da9b81404b1e0035c91755cc6ba497b.
Tested with JUCE's HelloWorld example with an added setUsingNativeTitleBar(true) in Main.cpp.