<pre> shouldn't render a final trailing newline

[jdm]

As part of the work in #26440, I discovered some surprising behaviour about <pre>. Given:

<pre>hello</pre>

<pre>hello
</pre>

<pre>hello

</pre>

In other browsers, only the final example with two newlines renders an empty line below hello. In the initial implementation in #26447, Servo renders an empty line for the second case as well. I have not been able to find spec text in https://drafts.csswg.org/css-text-3/ that explains why other browsers behave the way they do.

I've been reading:

• https://drafts.csswg.org/css-text-3/#white-space-processing (and 4.1)
• https://drafts.csswg.org/css-text-3/#line-breaking (and 5.1)

cc @SimonSapin

[SimonSapin]

I’ll look some more, but the first thing I tried is the same happens with <div> + white-space: pre instead of <pre>. This excludes anything at the HTML parser level and indicates that this is happening at the CSS layout level.

http://software.hixie.ch/utilities/js/live-dom-viewer/saved/8160

[SimonSapin]

It also happens with other kinds of forced line breaks. At most one trailing break is removed/ignored.

http://software.hixie.ch/utilities/js/live-dom-viewer/saved/8163

<p>hello</p>
<p>hello<br></p>
<p>hello<br><br></p>
<style>p { border: solid