Bug 1645817 - Use overlay scrollbars win browser resolution test. r=kats

Using overlay scrollbars will avoid the need to account for the trimmed
space that is introduced after an overflowing zoom caused by scrollbars.

Differential Revision: https://phabricator.services.mozilla.com/D80102

accessible/generic/Accessible.cpp

@@ -646,8 +646,8 @@ nsRect Accessible::BoundsInAppUnits() const {
   unionRectTwips.MoveBy(-viewportOffset);
 
   // We need to take into account a non-1 resolution set on the presshell.
-  // This happens in mobile platforms with async pinch zooming. Here we
-  // scale the bounds before adding the screen-relative offset.
+  // This happens with async pinch zooming. Here we scale the bounds before
+  // adding the screen-relative offset.
   unionRectTwips.ScaleRoundOut(presShell->GetResolution());
   // We have the union of the rectangle, now we need to put it in absolute
   // screen coords.

accessible/tests/browser/bounds/browser_test_resolution.js

@@ -45,6 +45,11 @@ async function testScaledBounds(browser, accDoc, scale, id, type = "object") {
 }
 
 async function runTests(browser, accDoc) {
+  // The scrollbars get in the way of container bounds calculation.
+  await SpecialPowers.pushPrefEnv({
+    set: [["ui.useOverlayScrollbars", 1]],
+  });
+
   await testScaledBounds(browser, accDoc, 2.0, "p1");
   await testScaledBounds(browser, accDoc, 0.5, "p2");
   await testScaledBounds(browser, accDoc, 3.5, "b1");