When L-shaped black screen is displayed, the zoom level percentage on the web browser is 125%.
If the zoom level percentage is changed to 100% on the web browser,
normal screen (no L-shaped black screen) is displayed.
*In case of Internet Explorer (IE)
If a slider of the zoom level is not displayed in the bottom right of IE, try to do as follow;
1) Press "Alt" key on the IE in order to display the Menu Bar
2) Select "View" - "Tool Bar" on the Menu Bar and check "Status Bar",
a slider of the zoom level is displayed in the bottom right of IE.