Add scrollbar.showScrollbar

Fixes #5676

src/browser/scrollable/scrollbarState.ts

@@ -35,7 +35,7 @@ export class ScrollbarState {
    * For the vertical scrollbar: the height of the scrollbar's arrows.
    * For the horizontal scrollbar: the width of the scrollbar's arrows.
    */
-  private readonly _arrowSize: number;
+  private _arrowSize: number;
 
   // --- variables
   /**
@@ -127,6 +127,14 @@ export class ScrollbarState {
     this._scrollbarSize = Math.round(scrollbarSize);
   }
 
+  public setArrowSize(arrowSize: number): void {
+    const iArrowSize = Math.round(arrowSize);
+    if (this._arrowSize !== iArrowSize) {
+      this._arrowSize = iArrowSize;
+      this._refreshComputedValues();
+    }
+  }
+
   public setOppositeScrollbarSize(oppositeScrollbarSize: number): void {
     this._oppositeScrollbarSize = Math.round(oppositeScrollbarSize);
   }