Center windows correctly, even if the windows don't start at 0,0

This is particularly noticeable when state has been rehydrated from a serialized state.

js/components/WindowManager.js

@@ -64,12 +64,15 @@ class WindowManager extends React.Component {
       const boxWidth = bounding.right - bounding.left;
 
       const move = {
-        x: Math.ceil(offsetLeft + (width - boxWidth) / 2),
-        y: Math.ceil(offsetTop + (height - boxHeight) / 2)
+        x: Math.ceil(offsetLeft - bounding.left + (width - boxWidth) / 2),
+        y: Math.ceil(offsetTop - bounding.top + (height - boxHeight) / 2)
       };
 
       const newPositions = this.props.windowsInfo.reduce(
-        (pos, w) => ({ ...pos, [w.key]: { x: move.x + w.x, y: move.y + w.y } }),
+        (pos, w) => ({
+          ...pos,
+          [w.key]: { x: move.x + w.x, y: move.y + w.y }
+        }),
         {}
       );
 

js/utils.ts

@@ -328,15 +328,19 @@ interface Window {
 }
 
 export const calculateBoundingBox = (windows: Window[]) =>
-  windows.reduce(
-    (b, w) => ({
-      left: Math.min(b.left, w.x),
-      top: Math.min(b.top, w.y),
-      bottom: Math.max(b.bottom, w.y + w.height),
-      right: Math.max(b.right, w.x + w.width)
-    }),
-    { top: 0, bottom: 0, left: 0, right: 0 }
-  );
+  windows
+    .map(w => ({
+      left: w.x,
+      top: w.y,
+      bottom: w.y + w.height,
+      right: w.x + w.width
+    }))
+    .reduce((b, w) => ({
+      left: Math.min(b.left, w.left),
+      top: Math.min(b.top, w.top),
+      bottom: Math.max(b.bottom, w.bottom),
+      right: Math.max(b.right, w.right)
+    }));
 
 export function findLastIndex<T>(arr: T[], cb: (val: T) => boolean) {
   for (let i = arr.length - 1; i >= 0; i--) {