Don't run light dismiss on right click

This fixes a bug where showing a popover during the contextmenu event
handler immediately gets light dismissed as soon as the user lifts up on
the mouse button because the contextmenu event is fired in between
pointerdown and pointerup.

Fixes whatwg#10905

Author: josepharhar

source

@@ -3359,6 +3359,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
     <ul class="brief">
      <li>The <dfn data-x-href="https://w3c.github.io/uievents/#mouseevent"><code>MouseEvent</code></dfn> interface</li>
      <li>The <code>MouseEvent</code> interface's <dfn data-x="dom-MouseEvent-relatedTarget" data-x-href="https://w3c.github.io/uievents/#dom-mouseevent-relatedtarget"><code>relatedTarget</code></dfn> attribute</li>
+     <li>The <code>MouseEvent</code> interface's <dfn data-x="dom-MouseEvent-button" data-x-href="https://w3c.github.io/uievents/#dom-mouseevent-button"><code>button</code></dfn> attribute</li>
      <li><dfn data-x-href="https://w3c.github.io/uievents/#dictdef-mouseeventinit"><code>MouseEventInit</code></dfn> dictionary type</li>
 
      <li>The <dfn data-x-href="https://w3c.github.io/uievents/#focusevent"><code>FocusEvent</code></dfn> interface</li>
@@ -62433,6 +62434,9 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
   <var>event</var>:</p>
 
   <ol>
+   <li><p>If <var>event</var>'s <code data-x="dom-MouseEvent-button">button</code> is not 0, then
+   return.</p></li>
+
    <li><p>Run <span>light dismiss open popovers</span> with <var>event</var>.</p></li>
 
    <li><p>Run <span>light dismiss open dialogs</span> with <var>event</var>.</p></li>