bug 1492499: marionette: bail if visibilitychange does not fire on minimization; r=whimboo

The visibilitychange DOM event does not fire reliably in all
configurations when retrieved from web content.  It appears to fire more
reliably in chrome, but to ensure a call to WebDriver:MinimizeWindow
never hangs, we additionally change it to be a TimedPromise.

There is an assumption here that if the iconification process does
not complete within this duration, there is nothing we can do.

Depends on D8413

Differential Revision: https://phabricator.services.mozilla.com/D8414

--HG--
extra : moz-landing-system : lando

testing/marionette/driver.js

@@ -3000,15 +3000,15 @@ GeckoDriver.prototype.minimizeWindow = async function() {
   const win = assert.open(this.getCurrentWindow());
   await this._handleUserPrompts();
 
-  if (WindowState.from(win.windowState) == WindowState.Fullscreen) {
-    await exitFullscreen(win);
-  }
-
   if (WindowState.from(win.windowState) != WindowState.Minimized) {
-    await new Promise(resolve => {
+    if (WindowState.from(win.windowState) == WindowState.Fullscreen) {
-      this.curBrowser.eventObserver.addEventListener("visibilitychange", resolve, {once: true});
+      await exitFullscreen(win);
+    }
+
+    await new TimedPromise(resolve => {
+      win.addEventListener("visibilitychange", resolve, {once: true});
       win.minimize();
-    });
+    }, {throws: null});
   }
 
   return this.curBrowser.rect;