In the Toolbar, you will find a button that looks like a hashmark. It allows you to turn on structure view. Specifically, it has 3 modes.
Right now, it’s off.
Clicking it will turn it into hover-only mode. That helps you see the page structure, by highlighting the file under the mouse. You will be able to judge the size of the content, which is of great help when you actively work on the page.
The 3rd mode is always-on, which highlights all files regardless of your mouse position, and also emphasizes the file when you hover your mouse over it.