Keep an explicit "isStacked" flag on metawindows + stackoverlay fixes

onCompleted of the tween of the active window didn't always finish before all
other windows was done animating (seemingly at least). This caused a wrong
overlay position/width since it used `is_scaled` to find the top of the stack.

In general it's probably useful to be able to know which windows are stacked
before animations are done. Windows might be scaled for other reasons too.

Also fixes a bug where the left overlay was active even though the neighbor
fully obscured the left stack. (that caused a small region of the left most
unstacked window to unresponsive). (due to setting negative width being a noop)

stackoverlay.js

@@ -64,8 +64,9 @@ var StackOverlay = new Lang.Class({
         let panelBox = Main.layoutManager.panelBox;
 
         overlay.y = panelBox.height;
-        overlay.width = stack_margin;
+        // global.window_group is below the panel so not really necessary to adjust height?
         overlay.height = this.monitor.height - panelBox.height; 
+        overlay.width = stack_margin;
 
         overlay.hide();
 
@@ -155,7 +156,7 @@ var StackOverlay = new Lang.Class({
             overlay.x = 0;
             overlay.width = Math.min(
                 stack_margin,
-                neighbourX - resizeBorderWidth
+                Math.max(0, neighbourX - resizeBorderWidth)
             );
         } else {
             let neighbour = space[space.indexOf(metaWindow) - 1]

tiling.js

@@ -120,7 +120,7 @@ focus = () => {
 }
 
 isStacked = function(metaWindow) {
-    return metaWindow.get_compositor_private().is_scaled();
+    return metaWindow._isStacked;
 }
 
 isUnStacked = function(metaWindow) {
@@ -158,6 +158,9 @@ move = (meta_window, x, y, onComplete, onStart, delay, transition) => {
         y += scaled_y_offset;
         let pivot = actor.pivot_point;
         actor.set_pivot_point(pivot.x, y_offset/buffer.height);
+        meta_window._isStacked = true; // use isStacked function to check
+    } else {
+        meta_window._isStacked = false; // use isStacked function to check
     }
     meta_window.destinationX = x;
     Tweener.addTween(actor, {x: x - x_offset
@@ -363,6 +366,7 @@ ensure_viewport = (space, meta_window, force) => {
             { x, y, delay, transition,
               onComplete: () => {
                   space.moving = false;
+                  // NOTE: Stack overlays should really be updated on the early returns above too
                   StackOverlay.leftOverlay.setTarget(space.topOfLeftStack());
                   StackOverlay.rightOverlay.setTarget(space.topOfRightStack());
                   // Certain gnome-shell/mutter animations expect default