Try the attached nb project. Launch HelloLabelValuePane.
Resize the window to double its width, then click on the "Stroke Type" CheckBox. The TitledPanes width is ~half the size of the window, when it should fill it. Now resize again the window : the layout is ok : the TitledPane fill correctly the window.

After investigation, this is related to TitledPane prefWidth (which is not set btw). If we set the prefWidth to Long.MAX_VALUE, this workaround the issue. You can check this by uncommenting line 91 of LabelValueBun.java.
To me, prefWidth should not interfere in TitledPane Layout in this case.