Correct vertical position of tabbed windows

Currently when the gap is zero, windows below tabs are repositioned
slightly downward. This causes the windows to exceed the bottom of the
screen and results in a small gap between the window and tabs. This
repositioning is unnecessary, so remove it.

Resolves #391

lib/extension/tree.js

@@ -1511,7 +1511,6 @@ export class Tree extends Node {
 
           if (gap === 0) {
             adjustY = renderRect.y;
-            nodeY = renderRect.y + params.stackedHeight + adjust / 4;
           }
 
           let decoration = node.decoration;