[css-text] What does the `white-space-collapse` apply to when white-space trimming/positioning

CSS-TEXT section 4.1.2 talks about the rendering and hanging of spaces in each line, and step 4 talks about whether a sequence of trailing spaces at the end of the line hangs, which depends on the values of white-space-collapse (and text-wrap-mode).

However, when you look at it in detail, it's not clear what value should be used here. This whole section applies to the line as a whole, and a sequence of trailing spaces can span inlines. Should then each preserved space in the sequence of the line's trailing spaces hang or not depending on its white-space-collapse value? This doesn't seem to make sense, since this would allow for some spaces to hang when not at the end of the line, which doesn't seem to make sense, and is not allowed by the definition of hanging.

<div style="white-space-collapse: continue">
foo&x#3000;&x#3000;&x#3000;<span style="white-space-collapse: break-spaces">        </span>
</div>

In this example, if the entirety of the text fits within the line, the ideographic spaces would hang (since they're not collapsed in phase I, so they count as preserved, but they're still white-space-collapse: continue), but the spaces inside the span wouldn't.

Because of this, when fixing hanging in Blink to align with the spec, I understood the white-space-collapse (and text-wrap-mode) values as applying to the line, rather than the spaces. Blink has a concept of a line box's style, which apparently is not in the spec (although I was not aware of that at the time of implementing this), which seems to be the same as the containing block box's computed style (except maybe including ::first-line styles) – and that is what I used in my implementation. This led to differences with other browsers in cases like this:

<div style="white-space-collapse: collapse; text-align: center">
Something.<span style="white-space-collapse: preserve; text-wrap-mode: wrap;">          </span>
</div>

If the relevant white-space-collapse (and text-wrap-mode) values should be those corresponding to each preserved space character, then the trailing spaces inside the span conditionally hang (since this is the last line), and therefore the "Something." text will be off-center (assuming that the text fully fits within the line, whether the spaces do or not). However, if the relevant values should be the containing block box's, then the spaces should hang (unconditionally) and the "Something." text will appear centered.

cc @kojiishi @jfkthame @fantasai @frivoal

Start with CSS-TEXT section 4.1.2 and its definition of hanging, then compare the two inline-style examples with Blink’s line-box behavior described in the issue. This needs a CSS Working Group decision on which styles govern trailing-space hanging; it is done when the specification’s wording and any relevant implementation behavior are resolved.