MDL-72874 behat: Allow behat window sizes to be modified during run

In some situations it is useful to modify the requested screensize by a
modifier. This allows for failing tests to be rerun at a different
screensize, where the original screensize triggered a failure for some
size-related reason.

config-dist.php

@@ -930,6 +930,12 @@ $CFG->admin = 'admin';
 // Example:
 //   $CFG->behat_increasetimeout = 3;
 //
+// Yon can specify a window size modifier for Behat, which is applied to any window szie changes.
+// For example, if a window size of 640x768 is specified, with a modifier of 2, then the final size is 1280x1536.
+// This is particularly useful for behat reruns to eliminate issues with window sizing.
+// Example:
+//   $CFG->behat_window_size_modifier = 1;
+//
 // Including feature files from directories outside the dirroot is possible if required. The setting
 // requires that the running user has executable permissions on all parent directories in the paths.
 // Example:

lib/behat/classes/behat_session_trait.php

@@ -742,6 +742,8 @@ trait behat_session_trait {
      * @throws ExpectationException
      */
     protected function resize_window($windowsize, $viewport = false) {
+        global $CFG;
+
         // Non JS don't support resize window.
         if (!$this->running_javascript()) {
             return;
@@ -769,6 +771,12 @@ trait behat_session_trait {
                 $width = (int) $size[0];
                 $height = (int) $size[1];
         }
+
+        if (isset($CFG->behat_window_size_modifier) && is_numeric($CFG->behat_window_size_modifier)) {
+            $width *= $CFG->behat_window_size_modifier;
+            $height *= $CFG->behat_window_size_modifier;
+        }
+
         if ($viewport) {
             // When setting viewport size, we set it so that the document width will be exactly
             // as specified, assuming that there is a vertical scrollbar. (In cases where there is