🎨: fix scrollExtent and scrollbarVisible prop when scrollbars hidden

Previously the hideScrollbars property would not be considered when calculating the scroll extent or the visibility of the scrollbar, leading to weird errors in calculating layouts on systems with larger scrollbars.

Author: merryman

lively.morphic/morph.js

@@ -1344,12 +1344,15 @@ export class Morph {
           to the rendered div container of a scrollable morph, such that HTML will end up with the
           same scrollable area as morphic, but that somehow does not work.
     */
-    return (this.submorphs.length
+    const scrollDefinedBySubmorphs = (this.submorphs.length
       ? this.innerBounds().union(this.submorphBounds())
-      : this.innerBounds()).extent().addPt(this.scrollbarOffset);
+      : this.innerBounds()).extent();
+    if (!this.hideScrollbars) { return scrollDefinedBySubmorphs.addPt(this.scrollbarOffset); }
+    return scrollDefinedBySubmorphs;
   }
 
   get scrollbarVisible () {
+    if (this.hideScrollbars) return { horizontal: false, vertical: false };
     if (this.clipMode === 'visible' || this.clipMode === 'hidden') return { horizontal: false, vertical: false };
     if (this.clipMode === 'scroll') return { horizontal: true, vertical: true };
     const extent = this.extent;