Are you sure Java's UI issues are caused by threading and not just Swing being a glitchy pile of junk?
For example, if you don't explicitly call the java.awt.Toolkit.sync() method after updating the UI state (which according to the docs "is useful for animation"), Swing will in my experience introduce seemingly random delays and UI lag because it just doesn't bother sending the UI updates to the window system.
only netbeans is written in swing . Eclipse and Jetbrains use their own thing and still generally struggled.
No, JetBrains use Swing in IntelliJ IDEA. You can tell from how it (for example) fails to layout dialogs correctly the first time they're displayed, just like every other Swing application. And how windows have no minimum size because Swing doesn't expose that functionality. And the various baffling bugs involving window focus that are inherent to Swing applications.
Eclipse uses SWT instead, which wraps the platform's native widgets.
When did you last use IntelliJ, 30 years ago? I've never seen it fail to lay out dialogs correctly, windows do have minimum sizes, and I haven't seen any focus bugs.