Fixes issue #2452, tooltips flicker when forced under the mouse

This scenario can occur when a tooltip is unable to render in the preferred
location due to the screen boundaries, for instance if an item with a really
long tooltip is placed in the lower-right most inventory slot.  Because UILists
define InteractionRegions around their contents by default and tooltips are
rendered last, they are seen as the new top-most interactive element on the
following game tick.  This causes an immediate reset of the tooltip timer due
to the top-most region changing, and the process repeats.  This has been fixed
by preventing tooltips from defining interaction regions through a new optional
flag in UIList that sets whether the list is interactive or not.

engine/src/main/java/org/terasology/rendering/nui/widgets/UIList.java

@@ -44,6 +44,7 @@ public class UIList<T> extends CoreWidget {
     private final List<ItemInteractionListener> itemListeners = Lists.newArrayList();
     private final List<ItemActivateEventListener<T>> activateListeners = Lists.newArrayList();
     private final List<ItemSelectEventListener<T>> selectionListeners = Lists.newArrayList();
+    private Binding<Boolean> interactive = new DefaultBinding<>(true);
     private Binding<Boolean> selectable = new DefaultBinding<>(true);
     private Binding<T> selection = new DefaultBinding<>();
     private Binding<List<T>> list = new DefaultBinding<>(new ArrayList<>());
@@ -80,7 +81,9 @@ public void onDraw(Canvas canvas) {
                 } else {
                     canvas.setMode(DEFAULT_MODE);
                 }
-                canvas.addInteractionRegion(listener, itemRenderer.getTooltip(item), itemRegion);
+                if (isInteractive()) {
+                    canvas.addInteractionRegion(listener, itemRenderer.getTooltip(item), itemRegion);
+                }
             } else {
                 canvas.setMode(DISABLED_MODE);
             }
@@ -134,10 +137,14 @@ public void bindSelectable(Binding<Boolean> binding) {
         selectable = binding;
     }
 
+    public boolean isInteractive() { return interactive.get(); }
+
     public boolean isSelectable() {
         return selectable.get();
     }
 
+    public void setInteractive(boolean value) { interactive.set(value); }
+
     public void setSelectable(boolean value) {
         selectable.set(value);
     }

modules/Core/src/main/java/org/terasology/rendering/nui/layers/ingame/inventory/ItemIcon.java

@@ -58,6 +58,7 @@ public class ItemIcon extends CoreWidget {
 
     public ItemIcon() {
         tooltip = new UIList<>();
+        tooltip.setInteractive(false);
         tooltip.setSelectable(false);
         final UISkin defaultSkin = Assets.getSkin("core:itemTooltip").get();
         tooltip.setSkin(defaultSkin);