Bug 1843227: When scrolling with nsIAccessibleScrollType::SCROLL_TYPE_ANYWHERE, scroll to center instead of nearest. r=eeejay

This is consistent with other browsers and thus more web compatible.
In particular, it makes infinite scroll work correctly on YouTube (e.g. when scrolling with screen readers), which didn't work previously.

Differential Revision: https://phabricator.services.mozilla.com/D185516

accessible/base/nsCoreUtils.cpp

@@ -312,9 +312,9 @@ void nsCoreUtils::ConvertScrollTypeToPercents(uint32_t aScrollType,
       whenX = WhenToScroll::Always;
       break;
     default:
-      whereY = WhereToScroll::Nearest;
+      whereY = WhereToScroll::Center;
       whenY = WhenToScroll::IfNotFullyVisible;
-      whereX = WhereToScroll::Nearest;
+      whereX = WhereToScroll::Center;
       whenX = WhenToScroll::IfNotFullyVisible;
   }
   *aVertical = ScrollAxis(whereY, whenY);

accessible/tests/browser/scroll/browser_test_scrollTo.js

@@ -4,33 +4,50 @@
 
 "use strict";
 
+function getCenterY(acc) {
+  const y = {};
+  const h = {};
+  acc.getBounds({}, y, {}, h);
+  return y.value + h.value / 2;
+}
+
 /**
  * Test nsIAccessible::scrollTo.
  */
 addAccessibleTask(
   `
-<div id="scroller" style="height: 1px; overflow: scroll;">
-  <p id="p1">a</p>
-  <p id="p2">b</p>
+<div id="scroller" style="height: 100vh; overflow: scroll;">
+  <hr style="height: 100vh;">
+  <p id="p1" style="height: 10vh;">a</p>
+  <hr style="height: 100vh;">
+  <p id="p2" style="height: 10vh;">b</p>
+  <hr style="height: 100vh;">
 </div>
   `,
   async function (browser, docAcc) {
     const scroller = findAccessibleChildByID(docAcc, "scroller");
-    // scroller can only fit one of p1 or p2, not both.
-    // p1 is on screen already.
-    const p2 = findAccessibleChildByID(docAcc, "p2");
-    info("scrollTo p2");
+    const scrollerY = getCenterY(scroller);
+    // scroller can only show one of p1 or p2, not both.
+    const p1 = findAccessibleChildByID(docAcc, "p1");
+    info("scrollTo p1");
     let scrolled = waitForEvent(
       nsIAccessibleEvent.EVENT_SCROLLING_END,
       scroller
     );
+    p1.scrollTo(SCROLL_TYPE_ANYWHERE);
+    await scrolled;
+    isWithin(getCenterY(p1), scrollerY, 10, "p1 scrolled to center");
+    const p2 = findAccessibleChildByID(docAcc, "p2");
+    info("scrollTo p2");
+    scrolled = waitForEvent(nsIAccessibleEvent.EVENT_SCROLLING_END, scroller);
     p2.scrollTo(SCROLL_TYPE_ANYWHERE);
     await scrolled;
-    const p1 = findAccessibleChildByID(docAcc, "p1");
+    isWithin(getCenterY(p2), scrollerY, 10, "p2 scrolled to center");
     info("scrollTo p1");
     scrolled = waitForEvent(nsIAccessibleEvent.EVENT_SCROLLING_END, scroller);
     p1.scrollTo(SCROLL_TYPE_ANYWHERE);
     await scrolled;
+    isWithin(getCenterY(p1), scrollerY, 10, "p1 scrolled to center");
   },
   { topLevel: true, iframe: true, remoteIframe: true, chrome: true }
 );