Added window size check upon p5 instantiation

This fixes a bug where `windowWidth` and `windowHeight` would represent the correct values when the window was resized and no p5 sketch was active

src/core/environment.js

@@ -675,7 +675,7 @@ p5.prototype.displayHeight = screen.height;
  * @alt
  * This example does not render anything.
  */
-p5.prototype.windowWidth = getWindowWidth();
+p5.prototype.windowWidth = 0;
 
 /**
  * A `Number` variable that stores the height of the browser's viewport.
@@ -703,7 +703,7 @@ p5.prototype.windowWidth = getWindowWidth();
  * @alt
  * This example does not render anything.
  */
-p5.prototype.windowHeight = getWindowHeight();
+p5.prototype.windowHeight = 0;
 
 /**
  * A function that's called when the browser window is resized.
@@ -800,6 +800,15 @@ function getWindowHeight() {
   );
 }
 
+/**
+ * Called upon each p5 instantiation instead of module import due to the
+ * possibility of the window being resized when no sketch is active.
+ */
+p5.prototype._updateWindowSize = function() {
+  this._setProperty('windowWidth', getWindowWidth());
+  this._setProperty('windowHeight', getWindowHeight());
+};
+
 /**
  * A `Number` variable that stores the width of the canvas in pixels.
  *

src/core/main.js

@@ -344,6 +344,8 @@ class p5 {
       this._events.devicemotion = null;
     }
 
+    this.registerMethod('init', this._updateWindowSize);
+
     // Function to invoke registered hooks before or after events such as preload, setup, and pre/post draw.
     p5.prototype.callRegisteredHooksFor = function (hookName) {
       const target = this || p5.prototype;