Copy a fairly long chunk of text (at least 100 words or so—the first several paragraphs of https://wikimediafoundation.org/wiki/FAQ/en will do fine) in Firefox. It will contain a number of single line breaks that do not exist in the source (but often come at or within a couple of words of the displayed line breaks). These breaks will persist when the content is pasted into VE; where they come right before a space, Parsoid will be forced to wrap the space in <nowiki> tags.
For example, the source HTML
<p><a class="external text" href="//www.wikipedia.org">Wikipedia</a> is the world's largest and most popular encyclopedia. It is online, free to use for any purpose, and free of advertising. Wikipedia contains more than 35 million volunteer-authored articles in over 288 languages, and is visited by more than 439 million people every month, making it one of the most popular sites in the world. </p> <p>It is a collaborative creation that has been added to and edited by millions of people during the past twelve years: anyone can edit it, at any time. It has become the largest collection of shared knowledge in human history. The people who support it are united by their love of learning, their intellectual curiosity, and their awareness that we know much more together than any of us does alone. </p>
is copied as:
<p><a class="external text" href="https://www.wikipedia.org">Wikipedia</a> is the world's largest and most popular encyclopedia. It is online, free to use for any purpose, and free of advertising. Wikipedia contains more than 35 million volunteer-authored articles in over 288 languages, and is visited by more than 439 million people every month, making it one of the most popular sites in the world. </p><p>It is a collaborative creation that has been added to and edited by millions of people during the past twelve years: anyone can edit it, at any time. It has become the largest collection of shared knowledge in human history. The people who support it are united by their love of learning, their intellectual curiosity, and their awareness that we know much more together than any of us does alone. </p>
which leads to the saved wikitext:
Wikipedia <nowiki> </nowiki>is the world's largest and most popular encyclopedia. It is online, free to use for any purpose, and free of advertising. Wikipedia contains <nowiki> </nowiki>more than 35 million volunteer-authored articles in over 288 languages, <nowiki> </nowiki>and is visited by more than 439 million people every month, making it one of the most popular sites in the world. It is a collaborative creation that has been added to and edited by millions of people during the past twelve years: anyone can edit it, at any time. It has become the largest collection of shared knowledge in <nowiki> </nowiki>human history. The people who support it are united by their love of learning, their intellectual curiosity, and their awareness that we know <nowiki> </nowiki>much more together than any of us does alone.