I had a look in the forums but couldn’t see anyone else reporting this bug.
When you go to export your file in Layout 2018, the Options button for export is stretched unnaturally hiding the Filetype options. Makes it impossible to select any other filetype other than whatever is selected as default - this as a significant issue! See attached screenshot.
Could be a High Sierra OSX problem. The screenshot is from my 2011 Macbook Pro.
You’re exactly correct that this issue is specific to High Sierra - we have a bug in the code that does the UI layout for this dialog. The workaround for now is to switch that dialog back to the “small” version by clicking on the drop arrow button to the right-hand side of the “Save as:” text box. For a more permanent fix you’d either need to edit one of the .nib files inside the LayOut installation or just wait for an update from us.