Thanks Mike, that's a useful suggestion, I'll give that a try.
Not sure why PL/SQL Developer leaves the statement highlighted after it has been run and it's a different "type" of highlighting compared to using the mouse/cursor to highlight text. Ideally, this highlight should be removed when clicking anywhere in the window, just like "normal" highlighting done manually using the mouse.
Yes, I have found that they have different types of windows, for different tasks and that some, like the Explain Plan Window can only contain a single statement. This is where the ease of use of TOAD is far superior to PL/SQL Developer, but I will persevere!