When creating or testing a Drupal theme, you may wish to know the current width and height of the browser viewport — especially when working on a responsive theme, for which it is critical to see the various break points of the window width as the browser is resized.
The WindowSize module can be quite helpful, by adding a small window at the top left corner of your web browser, in which is displayed the current window dimensions, dynamically recalculated with each browser size adjustment.
In the screenshot, the viewport has a width of 1264 pixels, and a height of 904. The WindowSize display does obscure the breadcrumbs, but in most cases that is inconsequential. By default, the "View WindowSize" permission is enabled only for the administrator.