Hi @Dweller, thanks for taking the time to report this!

Could you elaborate what exactly this report is about? That when editing wikitext directly (instead of e.g. using VisualEditor), an explicit blank line is needed to make the rendered wikitext display a line break?
Would the expected behavior be to display a line break even when there is no explicit blank line? Should that only apply to threaded discussions, or be a general behavior on any wikitext page?

Hi. Thanks for the swift and considered attention. I struggle for the terminology to explain myself. When replying to a comment, it seems that two presses of return are needed in order for the software to register one.

When replying to this line
Pressing return should make the next line appear below it

...but it doesn't. Hope that makes it clear. --~~~~

Another example from this morning: [https://en.wikipedia.org/w/index.php?title=Wikipedia:Reference_desk/Miscellaneous&diff=prev&oldid=669177064] --~~~~

See https://en.wikipedia.org/wiki/Help:Wiki_markup#Line_breaks :

A line break that is visible in the content is inserted by pressing Return twice.
Pressing Return once will place a line break in the markup, but it will not show in the rendered content, except when using list markup.

I don't think that is easy to solve because you might want to structure your raw wikitext via line breaks, without necessarily showing these line breaks in the output (the article shown in your browser).
Still this could be confusing but a better long-term way forward might be using a "What you see is what you get" text editor like VisualEditor.

Hi. Thanks. Why is this a desirable feature:
"A line break that is visible in the content is inserted by pressing Return twice.
Pressing Return once will place a line break in the markup, but it will not show in the rendered content, except when using list markup."
Why would anyone want to press return once and it not actually show up in the rendered content?

Might be a question for the discussion page of https://en.wikipedia.org/wiki/Help:Wiki_markup

Hi Dweller, IIUC, wikitext ignores a single linebreak in the markup in order to allow editors to use linebreaks for greater readability (in the edit-window) of things within a paragraph like templates (e.g. citation templates, where some editors prefer to use linebreaks before each | pipe),
and also (perhaps mainly) so that double-linebreaks can be used for very easily distinguishing any individual paragraphs in the edit-window.

The main situation where this breaks down (as you've given in your example diff) is in the root level replies in talkpages/discussions. This is one of many examples of how wikitext has never been an ideal way to structure discussions. Another good example is how we're mis-using the ::::colon as an indent. The colon is saved as <dd>content here</dd> in HTML (dd= definition-definition) and was always/originally intended to be used in conjunction with the semi-colon (which is saved as <dt>definition-term</dt>); see the 5th section of https://meta.wikimedia.org/wiki/Help:List#List_basics for a clear example.
: Hence some screenreaders (eg NVDA, but not JAWS (1)) speaks each new line as "definition. Hence some screenreaders [...]". Also, adding an empty line between indented replies causes problems in all screenreaders (see https://en.wikipedia.org/wiki/WP:LISTGAP for details).
The long-term solution for those problems, and the example that you've given, and many other problems, is the Flow software extension. (See mw:Flow for details, and ping me on the talkpage there, or my usertalkpage, if you have questions about it).

Hope that helps. I'm going to close this phabricator task, due to the nigh-impossibility of changing this aspect of mediawiki's page-editing ("it's not a bug, it's a feature!"). :-)