Toolbox App: Toolbox doesn't show up on Linux after clicking on the tray icon

Problem

On Linux, the Toolbox App tray icon appears, but the Toolbox App window never opens. Clicking the tray icon or the tray menu command to open the Toolbox App has no effect. No errors shown, no related exceptions in the logs. Restarting the Toolbox App or the machine does not help.

Reported environments include Ubuntu 26.04 with GNOME, Pop!_OS 24.04 with COSMIC, and Zorin OS based on Ubuntu 24.04.

A thread dump of the Toolbox App process shows the UI thread in native font manager initialization, and it stays there for minutes at full load on one CPU core:

"AWT-EventQueue-0" ... java.lang.Thread.State: RUNNABLE
	at org.jetbrains.skia.FontMgrKt._nDefault(Native Method)
	at org.jetbrains.skia.FontMgr.<clinit>(FontMgr.kt:13)
	...
	at com.jetbrains.toolbox.ui.window.TrayWidgetPreloader$preloadWidget$3.invokeSuspend(TrayWidgetPreloader.kt:58)

Cause

On Linux, the Toolbox App UI loads the system font list through the fontconfig library on the UI thread. When the fontconfig cache is stale or broken, fontconfig rescans and parses every font file before it returns. The UI thread cannot draw the window until this finishes, so the window never appears.

Toolbox 3.8.0.87617 - Not showing on Linux due to a stale fontconfig cache tracks this behavior in the Toolbox App.

Resolution

1. Ensure that the system fc-cache isn't shadowed by a copy from another environment:

which fc-cache

If the output is not /usr/bin/fc-cache, another environment might be shadowing the system tool. Anaconda is a known case: it ships its own fontconfig, and its fc-cache does not rebuild the system cache. Use

which -a fc-cache

...to see all fc-cache locations and select the one shipped with the OS.

2. Rebuild the fontconfig cache:

fc-cache -fv

...or use the full path to the system tool, located on the previous step.

3. Quit the Toolbox App from the tray icon menu.
4. Start the Toolbox App again.

If the window still does not open, collect logs and thread dumps. Attach them to a new report to the Toolbox App issue tracker.