In prior versions of Webix before 9.2, webix.env.isFF and related tests would return false if they were not true. In 9.2, they are undefined if they are not true. It is very hard to construct a valid expression that works when variables are either undefined or true. They should return true or false, not undefined.
See Code Snippet and try it with various web browsers under 9.2 and earlier versions of Webix.
Also, webix.env.isEdge does not return true when running the current version of Edge on either Mac or Windows. It always returns undefined. The webix.env.isChromium flag does return true, which helps, but the isEdge flag should return true as well. If this flag is intended for the old Edge web browser prior to the new Chromium based one, the documentation should say that.