Bug 1618678 - Make scrollbar style caching work on Android. r=heycam

Differential Revision: https://phabricator.services.mozilla.com/D65915

--HG--
extra : moz-landing-system : lando

layout/base/nsCSSFrameConstructor.cpp

@@ -3863,7 +3863,8 @@ nsresult nsCSSFrameConstructor::GetAnonymousContent(
   // * when visibility or pointer-events is anything other than the initial
   //   value; we rely on visibility and pointer-events inheriting into anonymous
   //   content, but don't bother adding this state to the AnonymousContentKey,
-  //   since it's not so common
+  //   since it's not so common. Note that on android scrollbars always have
+  //   pointer-events: none so we don't need to check for that.
   //
   // * when the medium is anything other than screen; some UA style sheet rules
   //   apply in e.g. print medium, and will give different results from the
@@ -3871,7 +3872,9 @@ nsresult nsCSSFrameConstructor::GetAnonymousContent(
   bool allowStyleCaching =
       StaticPrefs::layout_css_cached_scrollbar_styles_enabled() &&
       aParentFrame->StyleVisibility()->mVisible == StyleVisibility::Visible &&
+#ifndef ANDROID
       aParentFrame->StyleUI()->mPointerEvents == StylePointerEvents::Auto &&
+#endif
       mPresShell->GetPresContext()->Medium() == nsGkAtoms::screen;
 
   // Compute styles for the anonymous content tree.
@@ -3925,6 +3928,9 @@ nsresult nsCSSFrameConstructor::GetAnonymousContent(
             cachedStyles[i]->EqualForCachedAnonymousContentStyle(*cs),
             "cached anonymous content styles should be identical to those we "
             "would compute normally");
+#ifdef ANDROID
+        MOZ_ASSERT(cs->StyleUI()->mPointerEvents == StylePointerEvents::None);
+#endif
 #endif
         Servo_SetExplicitStyle(elements[i], cachedStyles[i]);
       }

toolkit/themes/mobile/global/scrollbars.css

@@ -20,6 +20,13 @@ xul|scrollbar {
   display: block;
 }
 
+/* We don't show resizers on android, so they may as well have
+ * pointer-events: none to simplify scrollbar caching */
+xul|resizer,
+xul|scrollcorner {
+  pointer-events: none;
+}
+
 xul|scrollbar[orient="vertical"] {
   min-width: 6px;
   max-width: 6px;