If you don’t use the NetBeans IDE, please ignore this thread.
I see that Apache has released version 26 of NetBeans. I haven’t downloaded it yet.
While the JMonkeyEngine SDK is based on NetBeans, I realize only a handful of people here use it directly. If you’re one of them, I’d love to hear about your experiences with the new release.
I’m currently using NetBeans 25 on Linux and 22 on Windows.
The latest SDK (3.8.0) is also using Netbeans 25, so no help there, I’m afraid. 
I downloaded and installed the ZIP file (platform-independent binary). I quickly discovered that NB 26 doesn’t recognize some of my projects, apparently those that have both “build.gradle.kts” and “package.json”. This is a regression relative to NB 25.
I’m too tired to investigate further tonight. I plan to open an issue tomorrow.
… and today I’m unable to reproduce that issue. ¯_(ツ)_/¯