[css-align] fully specify the "Overflow and Scroll Positions" section

[dbaron]

I think the current Overflow and Scroll Positions section, whose entire normative contents are currently:

The content distribution properties also affect the initial scroll position, setting it to display the appropriate portion of the scrollable area. In other words, the scrollable overflow region is aligned relative to the viewport as specified by the content distribution properties. Additionally, so that the content remains in the scrollable overflow region in order to be scrollable, the scroll and auto values of overflow trigger safe content alignment always.

is insufficient for what it's trying to specify.

I think there are a number of issues here:

• In the first sentence, "also affect" doesn't say how it affects the initial scroll position. For example, what does baseline do? What does center do? Do safe and unsafe influence the decision?
  • I'd note that I think it's a bad idea for the initial scroll position not to be at the edge of the scrollport, since existing scrolling user interfaces can make it hard to get back to a position that's not at the edge.
• The second sentence appears to confuse viewport and scrollport.
• The third sentence appears to completely contradict the first two, at least as I understand them. In other words, there only exists an initial scroll position when the content overflows, so this forces content-alignment on anything that is scrollable with more than one possible scroll position to be content-aligned to start. That, in turn, seems to imply that the initial scroll position would always be at the start edge like it's always been.

[tabatkins]

1. It has the effect stated - it aligns the scrollable overflow region within the scrollport, for the purpose of finding the initial scroll position. You appear to be reading this as talking about, more or less, where the scrollOffset=0 position is - it's not. What we're trying to describe is what, for example, Chrome does for an overflowing column-reverse flexbox - it starts the element scrolled down to the bottom.

   There's def some cleanup we need to do here, tho - fantasai and I are discussing this now.

2. Yeah, it does. Will fix.

3. I think this is also caused by your confusion regarding our use of "initial scroll position" vs "0 scroll offset". But also: we've some cleanup to do here.

[fantasai]

Okay, we've edited the section to hopefully be more precise. Let us know if you have any further concerns! https://drafts.csswg.org/css-align/#overflow-scroll-position

[dbaron]

The section appears to be unchanged from what I originally quoted.

[tabatkins]

Ah, looks like I accidentally reverted this text when I was rearranging sections in e693915#diff-77b45f4d92eb4bee76c4242f1752be4c

[fantasai]

:(

[dbaron]

I relanded the above patch. The only difference was the terminology change from "content distribution" to "content-distribution" (now hyphenated), which I made throughout (meaning that I made the substitution not only in the - lines of the patch which were necessary to make it apply, but also in the + lines of the patch and in the commit message).