I’ve created a custom component whose content can take up a lot of space. Is there a possibility to collapse the content in edit mode (via a property), so there’s more space for page development?
You could put in an if statement (toggle control) in your custom component that hides and shows the content?
@if(switch)
<li><b>This is only visible when switch is true</b></li>
@endif
Check out Conditional statements in the Docs, and the Switch UI control.
Let me know if you need any more help!
@dan: That’s exactly what I was looking for. 
I have another question: in some custom components, I can only see the result in preview mode and not in edit mode. Is there a solution to make the components visible in edit mode as well, and not just in preview mode?