[css-align] Make smart-unsafe the default behavior for overflow alignment, per WG resolution (SF May 2016)

Author: fantasai

css-align/Overview.bs

@@ -17,7 +17,7 @@ Previous Version: https://www.w3.org/TR/2012/WD-css3-align-20120612/
 Abstract: This module contains the features of <a href="https://www.w3.org/TR/CSS/">CSS</a> relating to the alignment of boxes within their containers in the various CSS box layout models: block layout, table layout, flex layout, and grid layout. (The alignment of text and inline-level content is defined in [[CSS-TEXT-3]] and [[CSS-INLINE-3]].)
 Link Defaults: selectors-3 (dfn) first formatted line, css-flexbox-1 (dfn) flex line, css-multicol-1 (dfn) multi-column element, css-fonts-3 (dfn) first available font, css21 (property) max-width/max-height/min-width/min-height, css-writing-modes-3 (dfn) dominant baseline, css-position-3 (property) left, css-cascade-3 (value) initial, css-flexbox-1 (dfn) static-position rectangle
 Ignored Terms: table cell, stretch, scrollable overflow region
-At Risk: ''last-baseline''
+At Risk: ''last-baseline'', the <<overflow-position>> keywords, the scrollable-area safety trigger into ''safe'' mode when no <<overflow-position>> is specified
 </pre>
 
 <style type="text/css">
@@ -400,19 +400,8 @@ Overflow Alignment: the ''safe'' and ''unsafe'' keywords</h3>
 
 	To help combat this problem,
 	an <dfn export>overflow alignment</dfn> mode can be explicitly specified.
-	"Unsafe" alignment honors the specified alignment mode in overflow situations, even if it causes data loss,
-	while "safe" alignment changes the alignment mode in overflow situations in an attempt to avoid data loss.
-
-	If the <a>overflow alignment</a> isn't explicitly specified,
-	the default <a>overflow alignment</a> is ''unsafe''.
-
-	Issue: This creates problems for people with small windows
-	since overflow to the start direction is unreachable.
-	We are considering having an omitted overflow alignment indicate a “smart” safe behavior,
-	which would true-align the alignment subject
-	unless it would overflow the nearest <em>scroll container</em>,
-	at which point it would cease to shift startward
-	so that it continued to remain within the scrollable area.
+	“Unsafe” alignment honors the specified alignment mode in overflow situations, even if it causes data loss,
+	while “safe” alignment changes the alignment mode in overflow situations in an attempt to avoid data loss.
 
 	<pre class='prod'><dfn>&lt;overflow-position></dfn> = unsafe | safe</pre>
 
@@ -428,11 +417,17 @@ Overflow Alignment: the ''safe'' and ''unsafe'' keywords</h3>
 			the given alignment value is honored.
 	</dl>
 
-	Issue: Should we add a third (default) value that overflows up until the point that it would overflow into an unscrollable region?
-	This is probably what's actually needed;
-	we can't default to "safe" because of Flexbox,
-	but this slight tweak to "unsafe" is probably safe compat-wise,
-	and would help users on small screens.
+	If the <a>overflow alignment</a> isn't explicitly specified,
+	the default <a>overflow alignment</a> is similar to ''unsafe''
+	in that an overflowing <a>alignment subject</a> is allowed to overflow its <a>alignment container</a>;
+	however if the this would cause it to also overflow
+	the <a>scrollable overflow region</a> of its nearest ancestor <a>scroll container</a>,
+	then its overflow in that direction is limited
+	by biasing any remaining overflow to the end side.
+	In other words, it is similar to ''safe'' alignment
+	except that the limit for switching to ''start''
+	is triggered by overflowing the scrollable area,
+	rather than merely overflowing the <a>alignment container</a>.
 
 	<div class='example'>
 		The figure below illustrates the difference in "safe" versus "unsafe" centering,
@@ -488,6 +483,10 @@ Overflow Alignment: the ''safe'' and ''unsafe'' keywords</h3>
 		</figure>
 	</div>
 
+	Issue: It may not be Web-compatible to implement the “smart” default behavior
+	(though we hope so, and believe it to be likely),
+	so UAs should pass any feedback on this point to the WG.
+
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