MOBILE-4270 behat: Fix default window size

Tests were already running with these dimensions, but we hadn't noticed because unless the tests are run in headless mode, the minimum width is 500px.

local_moodleappbehat/tests/behat/behat_app.php

@@ -45,7 +45,6 @@ class behat_app extends behat_app_helper {
     ];
 
     protected $featurepath = '';
-    protected $windowsize = '360x720';
 
     /**
      * @BeforeScenario

local_moodleappbehat/tests/behat/behat_app_helper.php

@@ -201,7 +201,7 @@ class behat_app_helper extends behat_base {
             $restart = true;
 
             // Reset its size.
-            $this->resize_window($this->windowsize, true);
+            $this->resize_app_window();
 
             // Visit the Ionic URL.
             $this->getSession()->visit($this->get_app_url());
@@ -596,4 +596,18 @@ EOF;
 
         return null;
     }
+
+    /**
+     * Resize window to have app dimensions.
+     */
+    protected function resize_app_window() {
+        $width = 500;
+        $height = 720;
+        $offset = $this->evaluate_script("{
+            x: window.outerWidth - document.body.offsetWidth,
+            y: window.outerHeight - window.innerHeight,
+        }");
+
+        $this->getSession()->getDriver()->resizeWindow($width + $offset['x'], $height + $offset['y']);
+    }
 }