If you have TabPanes in TabPanes and disable an overlying TabPane it cascades down through all TabPanes and the reduced opacity cumulates unti nothing can be seen anymore. This even affects the own Tabs of the disabled TabPane (see attached image).

As a workaround we overwrite the modena style ".tab-pane:disabled { -fx-opacity: 0.4; }" with ".tab-pane:disabled { -fx-opacity: 1.0; }" and everything looks as expected.

Maybe ".tab-pane:disabled" should be removed from disabled-section in the modena stylesheet. The ".tab-pane > .tab-header-area > .headers-region > .tab:disabled" is still there and should result in the desired look (see attached image).