It’s a third party editing program Paul. If you post a long link it sees it as one word which won’t fit into the available space between the outside columns. Instead of splitting the link, it widens the window to the right which takes it underneath the adverts. It’s to do with the way the site is constructed.
As far as the “Insert Link” facility is concerned, there are a number of options which are not relevant to just inserting a link into a forum post although some of them are used elsewhere. You have to remember that the Model Boats site is not just a Forum, it’s a general purpose site with a forum included so I’m afraid we have to put up with these little quirks!
As long as the link will fit between the flanking columns, as most do, there isn’t a problem. It’s only the very long ones which cause the difficulty.
No doubt things could be tailored to improve ease of use but that would cost time and money…..
Colin