Starting with Xojo 2017r2, Linux apps use GTK3. Since Xojo uses native controls that means your app’s controls will use the theme of the Linux distribution the app runs on. This can sometimes mean that your app’s UI will not look exactly like what you designed in the Layout Editor because a theme may dramatically change control sizes and padding. This is a problem that can occur with any modern GTK3 app.
If this is a problem for your apps, there are a couple ways you can work around it.