[cssom-view] Change defaults for ScrollIntoViewOptions

Fixes #1720.

Author: zcorpan

cssom-view/Overview.bs

@@ -1075,14 +1075,14 @@ Note: This {{DOMRect}} object is not <a spec=html>live</a>.
 <pre class=idl>
 enum ScrollLogicalPosition { "start", "center", "end", "nearest" };
 dictionary ScrollIntoViewOptions : ScrollOptions {
-  ScrollLogicalPosition block = "center";
-  ScrollLogicalPosition inline = "center";
+  ScrollLogicalPosition block = "start";
+  ScrollLogicalPosition inline = "nearest";
 };
 
 partial interface Element {
   DOMRectList getClientRects();
   [NewObject] DOMRect getBoundingClientRect();
-  void scrollIntoView(optional (boolean or ScrollIntoViewOptions) arg = true);
+  void scrollIntoView(optional (boolean or ScrollIntoViewOptions) arg);
   void scroll(optional ScrollToOptions options);
   void scroll(unrestricted double x, unrestricted double y);
   void scrollTo(optional ScrollToOptions options);
@@ -1140,20 +1140,17 @@ The <dfn method for=Element caniuse=scrollintoview>scrollIntoView(<var>arg</var>
 1. Let <var>options</var> be null.
 1. If <var>arg</var> is a {{ScrollIntoViewOptions}} dictionary, set <var>options</var> to <var>arg</var>. Otherwise:
     1. <a lt="converted to an IDL value">Convert</a> <var>options</var> to a {{ScrollIntoViewOptions}} dictionary. [[!WEBIDL]]
-    1. If <var>arg</var> is true, set the {{ScrollIntoViewOptions/block}} dictionary member of <var>options</var> to "<code>start</code>", and set the {{ScrollIntoViewOptions/inline}} dictionary member of <var>options</var> to "<code>nearest</code>".
-    1. If <var>arg</var> is false, let the {{ScrollIntoViewOptions/block}} dictionary member of <var>options</var> to "<code>end</code>", and set the {{ScrollIntoViewOptions/inline}} dictionary member of <var>options</var> to "<code>nearest</code>".
+    1. If <var>arg</var> is false, let the {{ScrollIntoViewOptions/block}} dictionary member of <var>options</var> to "<code>end</code>".
 1. If the element does not have any associated <a>layout box</a> terminate these steps.
 1. <a lt='scroll an element into view'>Scroll the element into view</a>
     with the options <var>options</var>.
 1. Optionally perform some other action that brings the element to the user's attention.
 
-<p class=note highlight=javascript>For historical reasons, passing in <code>true</code> or
-<code>undefined</code> to {{Element/scrollIntoView()}} is equivalent to
-<code>{&nbsp;block:&nbsp;"start",&nbsp;inline:&nbsp;"nearest"&nbsp;}</code>, and passing in
+<p class=note highlight=javascript>Passing in <code>true</code>, <code>undefined</code>, or an empty
+dictionary <code>{}</code> to {{Element/scrollIntoView()}} is equivalent to
+<code>{&nbsp;block:&nbsp;"start",&nbsp;inline:&nbsp;"nearest"&nbsp;}</code>. Passing in
 <code>false</code> is equivalent to
-<code>{&nbsp;block:&nbsp;"end",&nbsp;inline:&nbsp;"nearest"&nbsp;}</code>. Since centering is a
-common case, passing in an empty dictionary <code>{}</code> is equivalent to
-<code>{&nbsp;block:&nbsp;"center",&nbsp;inline:&nbsp;"center"&nbsp;}</code>.
+<code>{&nbsp;block:&nbsp;"end",&nbsp;inline:&nbsp;"nearest"&nbsp;}</code>.
 
 The <dfn method for=Element lt="scroll(options)|scroll(x, y)">scroll()</dfn> method must run these steps: