(J)Wt 4.13.1 has just been released.
It is a patch release that fixes an issue with WSpinBox and WDoubleSpinBox. The issue was really problematic for JWt but had likely little impact on Wt.
Check the release notes for more info.
Here are the links:
Wt 4.13.1 (C++): download wt-4.13.1.tar.gz (release notes)
JWt 4.13.1 (Java): download jwt-4.13.1.zip (release notes)
Binary builds for Windows are available on the GitHub releases page.