There's a white box over my screen, so I can't see full window.

The zoom settings of the browser needs to be checked. Ensure that the value is not something bigger than 100%

To solve this, press CTRL + 0  (that's Zero key)  on your keyboard while your browser is open. This should set the zoom back to 100%.

Here is a video replicating the issue

If at 100% you are still not seeing your page and page contents (big white box) on the right, please try to make your screen even smaller - say 60%.