This Bugzilla instance is a read-only archive of historic NetBeans bug reports. To report a bug in NetBeans please follow the project's instructions for reporting issues.
There are actually 3 possible states of checkbox - checked, unchecked and semi-checked (values Boolean.TRUE, Boolean.FALSE and null). The semi-checked value is used e.g. in NB Module Suite project customizer, where it denotes clusters with excluded modules (see screenshot). Such checkbox is not aligned with checked/unchecked ones and hops left and right when clicked :).
Created attachment 80522 [details] screenshot
Reproduced, looks ugly.
This appears to be caused by the special handling for Boolean3WayEditor in org.openide.explorer.propertysheet.RendererFactory.delegatedPaint (see http://hg.netbeans.org/releases/file/b2429dbb1ff8/openide.explorer/src/org/openide/explorer/propertysheet/RendererFactory.java#l900).