Documentation tool window
When you use Ctrl+Q (View | Quick Documentation), the Documentation popup is shown.
Then, if you click in the upper-right corner of the popup, the popup transforms into a tool window. As a result, the tool window becomes available in the View | Tool Windows menu.
To turn the tool window back into a popup, click again.
Left or Right
Switch to the previous or go to the next documentation page (for example, after clicking hyperlinks).
On macOS, you can also use the three-finger right-to-left and left-to-right swipe gestures.
Navigate to the source code in the editor.
Enable and disable showing code documentation tool window as you hover the mouse over symbols.
When the Show on Mouse Move option is disabled, you have to explicitly open the tool window by pressing Ctrl+Q.