Same as Guillaume Hochart writes. Doesn't work under Vaadin Flow. Tried you

Directory HybridMenu
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Same as Guillaume Hochart writes. Doesn’t work under Vaadin Flow. Tried your HM-Demo cloned from vaadin10 branch: java.io.FileNotFoundException: file:/Users/xxx/projects/Hybrid-Menu/HM-Demo/target/HM-Demo-3.0.8.vaadin10/WEB-INF/lib/hybridmenu-3.0.8.vaadin10.jar!/css/hybridMenu.css (No such file or directory)

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The bug is fixed in 3.0.9.vaadin10