[css-anchor-position] Make base styles *not* part of the position options list, so it's not affected by position-try-order sorting.

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1025,7 +1025,7 @@ This property provides a list of alternate positioning styles
 to try when the [=absolutely positioned box=]
 overflows its [=inset-modified containing block=].
 This <dfn export>position options list</dfn>
-initially contains the element's own computed style.
+is initially empty.
 
 Each comma-separated entry in the list is a separate option:
 either the name of a ''@position-try'' block,
@@ -1037,8 +1037,7 @@ Values have the following meanings:
 	: <dfn>none</dfn>
 	::
 		The property has no effect;
-		the only entry in the [=position options list=]
-		is the element's computed style.
+		the element's [=position options list=] is empty.
 
 	: <dfn><<dashed-ident>></dfn>
 	::
@@ -1128,12 +1127,61 @@ the [=position options list=] will be tried.
 	: <dfn>most-block-size</dfn>
 	: <dfn>most-inline-size</dfn>
 	::
-		Stable sort the [=position options list=]
-		in order of greatest to least
-		[=inset-modified containing block=] size
-		in the matching axis.
+		For each entry in the [=position options list=],
+		[=determine the position fallback styles=],
+		and find the specified [=inset-modified containing block=] size
+		that results from those styles.
+		Stably sort the [=position options list=]
+		according to this size,
+		with the largest coming first.
 </dl>
 
+<div class=example>
+	For example, the following styles
+	will initially position the popup list below its anchoring button,
+	but if that overflows,
+	will decide whether to keep the popup list below the anchor
+	or move it above,
+	depending on which option gives it the most space.
+
+	<pre highlight=css>
+	.anchor { anchor-name: --foo; }
+	.list {
+		position: fixed;
+		anchor-default: --foo;
+		top: anchor(bottom);
+		left: anchor(left);
+		align-self: start;
+		position-try-options: --bottom-scrollable, flip-block, --top-scrollable;
+		position-try-order: most-height;
+	}
+	@position-try --bottom-scrollable {
+		top: anchor(bottom);
+		bottom: 0;
+		align-self: stretch;
+	}
+	@position-try --top-scrollable {
+		bottom: anchor(top);
+		top: 0;
+		align-self: stretch;
+	}
+	</pre>
+
+	The ''flip-block'' auto-generated option and the ''--top-scrollable'' option
+	will always find the same available height,
+	since both of them stretch vertically from ''top: 0''
+	(the top edge of the viewport)
+	to ''bottom: anchor(top)''
+	(the top of the anchor),
+	so they'll retain their specified order.
+	This causes the element to first try to align against the anchor
+	at its natural height
+	(using ''align-self: end'', auto-reversed from the base styles),
+	but if that also causes overflow,
+	it'll fall back to just filling the space
+	and being scrollable instead.
+</div>
+
 Final Fallback Strategy: the 'position-try-final' property {#position-try-final-property}
 ----------------------
 
@@ -1322,7 +1370,8 @@ Animation type: discrete
 Applying Position Fallback {#fallback-apply}
 --------------------------
 
-When an element uses a [=position options list=],
+When a positioned element overflows its [=inset-modified containing block=],
+and has a non-empty [=position options list=],
 it selects one entry from the list
 as defined below,
 and applies those properties to itself as [=used values=].