Everything has been typed in the box here. No external programs.
Unfortunately not, I've made a habit of quickly editing affected posts. That being said, this post will be an example of unintended triple-spacing.
I could have sworn everything worked correctly for the first week or so of active posting, but... I honestly can't be 100% sure. I've slept since then. I don't generally futz with the formatting buttons and dropdowns when I can help it, I'm an old BBCode grognard too used to just manually typing each tag. Curiously, the issue with deleting line breaks does shrink the text in the post/reply box as though I had selected a different size from the dropdown menu, and also seems to be, oddly enough, a fix for the triple-spacing issue (resetting the font size adds a (size=2)Lorem Ipsum(/size) tag to each paragraph, which I... suspect helps fix things? But said tags are only visible while editing afterwards.).
Also, these issues don't seem to appear when editing a post or reply, only creating one. Furthermore, these symptoms persist across multiple computers.
EDIT: Here's what my Look and Layout settings look like, as I suspect one of them may be the culprit. A lot of this, if memory serves, started up around the time I changed them (primarily to return to topic, as that's what I'm used to). That being said, I can't be certain.
The only difference between mine and yours was that I did not have the "Show WYSIWYG editor on post page by default" set to on. I'm going to change mine to be on, and see if I can get a similar problem. The first issue I can see is that you only see the final product, rather than the behind-the-scenes that goes on. For example, your linked image is a hyperlink, rather than the "url=" and "/url" (but with brackets) that I would see.
If you get another post with this issue, can you link to that post in this thread, or just copy-paste the post so we can try to get a better idea?
The other option might be for you to unselect the "Show WYSIWYG editor on post page by default" so you have better control over your posts.
One idea might be using the "Preview" button so you can see what your post will look like before posting. This combined with unselecting the WYSIWYG could help you see where the commands are causing issues. Preview is one of the major features I use on this forum, and if someone looks at my forum activity they would see that I use the "Preview" option multiple times on each post, to make sure I get it right (and even then I fail sometimes).
I tried testing by making a large paragraph after the above sentence, then pressing the "Delete" key to bring the large paragraph up, but was unable to get the "size=78%". The one thing I did notice is that the single line breaks I had in the text pane did not show up in the Preview. I had to put double line breaks to separate the individual sentences and paragraphs above.
