Like macOS and Windows, Linux has some specific concerns and idiosynchrasies that Jaunch must accommodate.
On systems running the X Window System, it might be necessary to invoke the XInitThreads() function before interacting with the display across multiple threads, for example when using OpenGL or Xlib for window management.
To accommodate this (admittedly niche) use case, Jaunch provides an INIT_THREADS directive, which calls Jaunch's init_threads() function, which on Linux invokes XInitThreads(). For other operating systems, init_threads() is currently a no-op.
The INIT_THREADS directive was originally included in Jaunch for one specific reason: to support the sciview plugin in Fiji. The sciview plugin is built on the scenery rendering library, which previously had an OpenGL backend, thus requiring an XInitThreads() call for multithreaded visualization to work. More recently, scenery dropped its OpenGL backend in favor of a Vulkan-based one, so the XInitThreads() call might not be necessary anymore for sciview and scenery. But Fiji has other plugins that still utilize OpenGL, and other applications might potentially need it as well, so Jaunch retains the INIT_THREADS feature.