[Linux] SWT 4.39 - Tree with size "preferred" disappears

[pbi-qfs]

Describe the bug
A tree control with size "preferred" is not visible or disappears when changing the style.

To Reproduce

• Install SWT Controls demo as described in https://eclipse.dev/eclipse/swt/examples.html
• Open Controls demo (standalone or as view in Eclipse)
• Select "Tree" tab -> Tree already is (almost invisible)
• Select a fixed size, then "preferred" again, or resize the window -> Tree is visible
• Toggle any style, w.g. SWT.CHECK -> Tree disappears

Expected behavior
The tree should be visible all the time.

Screenshots

Environment:

1. Select the platform(s) on which the behavior is seen:

• • All OS
• • Windows
• • Linux
• • macOS

2. Additional OS info (e.g. OS version, Linux Desktop, etc)

• Seen on: Ubuntu 18, 20, 24

3. JRE/JDK version

• JDK 21 (Tested: Zulu)
• JDK 25 (Tested: Zulu, Temurin, Oracle)

Version since
4.39M1, still seen in M2

Workaround (or) Additional context
Maybe unrelated: When opening the SWT Controls view in Eclipse, this error is logged six times in the console:

(Eclipse:37891): Gtk-WARNING **: 12:36:28.297: gtk_widget_size_allocate(): attempt to allocate widget with width -5 and height 0

[akurtakov]

It would be nice if you can spend the time to come with a minimal snippet to reproduce the issue. That would allow me to do more work on SWT.

[pbi-qfs]

We use the aforementioned default SWT Controls example from the SWT project source as testing base (https://github.com/eclipse-platform/eclipse.platform.swt/blob/master/examples/org.eclipse.swt.examples/src/org/eclipse/swt/examples/controlexample/TreeTab.java), so it should be easy to reproduce and track down using these official sources. I do not have any other source example, and I'm not familiar enough with the SWT sources to strip the controls example down.