I had my suspicions about copy/paste and almost mentioned it in the original post, but I didn't want to make the message too long if the HTML view didn't show anything unusual.
Here's a fuller explanation.
Let's say you're viewing content within Canvas and select/copy it so that you can paste it into the Rich Content Editor (RCE). When you hit copy, it includes extra HTML that was added by Canvas when displaying the information.
For example, let's say I have a very simple line of text that reads:
You must view the study guide and score a minimum of 5 out of 10 points on the review before you can take your final.
If you paste it into a text editor (like notepad), comes through exactly as expected.
However, if you paste it into the RCE, it comes through wrapped in a span element.
<span>You must view the study guide and score a minimum of 5 out of 10 points on the review before you can take your final.</span>
That is also most likely wrapped in a paragraph tag, but it could be inside a div or other block level element.
Since what you are copying is HTML content, Chrome makes sure that you're saving valid HTML to the clipboard. Valid HTML doesn't allow you to have text on its own, it has to be wrapped in some kind of element. Sometimes those are block level, like paragraph or div. But when it's not a block element, like a few words within a paragraph, then it gets wrapped with a span element.
Since you were copying just a single line of text, it got wrapped in a span element. Now whether or not that was the actual cause of the issue, I'm not sure. But I do know it causes me headaches to the point where I've written code that looks at my HTML and removes any span elements from it (except for certain ones I need).
This issue seems to be application specific. In Chrome, the span is there, but if I use Firefox to copy a portion of a line, then it does not wrap it in span. Even if I copy HTML (something that has hyperlinks in it for example), Firefox does a much better job of making it work. Firefox didn't include the span or add the non-breaking-spaces. Selecting/copying the same content in Chrome added both.
When I tried copying multiple lines, Chrome grabbed the paragraph elements that wrapped it. Since the content was valid HTML, it didn't need to wrap it further, and it worked okay.
If you are editing the content with the RCE, rather than viewing it, then Chrome doesn't pick up add the extra span. Because I mostly use Chrome, my long-time-ago-lesson-learned is that whenever I want to copy/paste parts of one page to another, I edit the first page. Then I select/copy the text. Then I can go and paste it getting it the way I wanted it.
Opening the Rich Content Editor just so you can get a clean paste for a single line of text wastes a lot of time. What I do for simple stuff is use Shift+Ctrl+V (Firefox or Chrome) or Command+Option+V (Safari) to paste as text only.
