How can I insert line breaks in text boxes when using Greenshot's image editor?
Usually, screenshot annotations are of a brief nature and thus don't need paragraphs and line breaks - this is why Enter and Return finish editing of a text box instead of inserting a line break, similar to other software e.g. MS Excel.
Of course, there are situations in which there is need to add line breaks. You can do so by simply hitting Shift + Return or Shift + Enter, respectively.