skip render if current view remains unchanged

Rebasing on top of !27.

js/src/main/scala/cs214/webapp/client/graphics/WebClientAppInstance.scala

@@ -13,6 +13,9 @@ abstract class WebClientAppInstance[Event, View](
     sendMessage: ujson.Value => Unit,
     target: Target
 ) extends StateMachineClientAppInstance[Event, View](userId, sendMessage), DOMClientUI[View], PatchableUI:
+  /** The currently-rendered view, if one exists. */
+  private var currentView: Option[View] = None
+
   /** Renders a [[View]] received from the server. The method also takes a
     * [[UserId]] to get information on the context and an [[onEvent]] callback
     * which it uses to send server events when specific actions are triggered on
@@ -27,7 +30,9 @@ abstract class WebClientAppInstance[Event, View](
   protected def render(userId: UserId, view: View): Frag
 
   override def displayView(view: View): Unit =
-    patchUI(target, render(userId, view))
+    if Some(view) != currentView then
+      currentView = Some(view)
+      patchUI(target, render(userId, view))
 
   override def patchUI(target: dom.Element, fr: Frag): Unit =
     target.replaceChildren(fr.render)