docking: Use an even width for the overview content area

For some reason we need to use an even value, this is probably due to
the fact that such value is going to be proportional to the available
space.

Closes: #1612
LP: #1979096

docking.js

@@ -2114,7 +2114,13 @@ var DockManager = class DashToDock_DockManager {
             if (this.mainDock.isHorizontal || this.settings.dockFixed)
                 return box;
 
-            const [, preferredWidth] = this.mainDock.get_preferred_width(box.get_height());
+            let [, preferredWidth] = this.mainDock.get_preferred_width(box.get_height());
+
+            // For some reason we need to use an even value for the box area
+            // or we may end up in allocation issues:
+            // https://github.com/micheleg/dash-to-dock/issues/1612
+            preferredWidth = Math.floor(preferredWidth / 2.0) * 2
+
             box.x2 -= preferredWidth;
             if (this.mainDock.position === St.Side.LEFT)
                 box.set_origin(box.x1 + preferredWidth, box.y1);