Components can have focus. This is analagous to HTML elements having focus.
Only one component in a layout can have focus at any time (or alternatively, no component has focus). Similarly to HTML elements, a component will be focused when you click on its tab. You can programatically give a component focus by calling the focus() method on its container. Likewise, you can remove focus from a container by calling ComponentContainer.blur().
Clicking on HTML within a component will not automatically give a Golden Layout component focus. However this can be achieved by listening to the bubbling click and/or focusin events and calling ComponentContainer.focus() in these events’ handlers. The apitest demonstrates this technique.
A focused component’s tab and header HTML elements will contain the class lm_focused. This can be used to highlight the focused tab and or header. The goldenlayout-dark-theme.less theme that ships with Golden Layout (and is used by apitest) will set the background color of a focused tab to a different color from other tabs. If you do NOT want focused tabs to be highlighted, ensure that the lm_focused selector is removed from the relevant css/less/scss used by your application.