In my emacs, basically all of this code has implicit underlining across the entire screen.

emacs-ensime-implicit-highlighting

   class Backend($: BackendScope[ModelProxy[DisplayModel], Unit]) {
     def render(proxy: ModelProxy[DisplayModel]) = {
       val x = Array[String]().toVector  // This highlights correctly

       <.div(
         proxy().metadata.render(md => {                                                                                                                                                                            
           <.div(                                                                                                                                                                                                   
             <.h3("Layer Metadata"),                                                                                                                                                                                
             <.p(s"CellType: ${md.cellType}"),                                                                                                                                                                      
             <.p(s"CRS: ${md.crs}"),                                                                                                                                                                                
             <.div(                                                                                                                                                                                                 
               <.h4("Layer Extent"),                                                                                                                                                                                
               <.div(                                                                                                                                                                                               
                 <.p(s"xmin: ${md.extent.xmin}"),                                                                                                                                                                   
                 <.p(s"xmin: ${md.extent.ymin}"),                                                                                                                                                                   
                 <.p(s"xmin: ${md.extent.xmax}"),                                                                                                                                                                   
                 <.p(s"xmin: ${md.extent.ymax}")                                                                                                                                                                    
               )                                                                                                                                                                                                    
             ),                                                                                                                                                                                                     
             <.h4("Bounds"),                                                                                                                                                                                        
             <.div(                                                                                                                                                                                                 
               <.p(s"MinKey: ${md.bounds.minKey.toString}"),                                                                                                                                                        
               <.p(s"MaxKey: ${md.bounds.maxKey.toString}")                                                                                                                                                         
             ),                                                                                                                                                                                                     
             <.div(                                                                                                                                                                                                 
               <.h4("Layout Definition"),                                                                                                                                                                           
               <.div(                                                                                                                                                                                               
                 <.h5("Definition Extent"),                                                                                                                                                                         
                 <.div(                                                                                                                                                                                             
                   <.p(s"xmin: ${md.layoutDefinition.extent.xmin}"),                                                                                                                                                
                   <.p(s"ymin: ${md.layoutDefinition.extent.ymin}"),                                                                                                                                                
                   <.p(s"xmax: ${md.layoutDefinition.extent.xmax}"),                                                                                                                                                
                   <.p(s"ymax: ${md.layoutDefinition.extent.ymax}")                                                                                                                                                 
                 ),                                                                                                                                                                                                 
                 <.h5("Tile Layout"),                                                                                                                                                                               
                 <.div(                                                                                                                                                                                             
                   <.p(s"Layout cols: ${md.layoutDefinition.tileLayout.layoutCols}"),                                                                                                                               
                   <.p(s"Layout rows: ${md.layoutDefinition.tileLayout.layoutRows}"),                                                                                                                               
                   <.p(s"Tile cols: ${md.layoutDefinition.tileLayout.tileCols}"),                                                                                                                                   
                   <.p(s"Tile rows: ${md.layoutDefinition.tileLayout.tileRows}")                                                                                                                                    
                 )                                                                                                                                                                                                  
               )                                                                                                                                                                                                    
             )                                                                                                                                                                                                      
           )                                                                                                                                                                                                        
         }),
         proxy().attributes.render(attrs => {                                                                                                                                                                       
           <.h3("Extra Attributes")                                                                                                                                                                                 
           attrs.attrs.map({ case (k,v) => <.p(s"${k}") })                                                                                                                                                          
         })                                                                                                                                                                                                         
       )
     }
   }