Bug 1722359 - Remove ui.showHideScrollbars. r=mstange

It probably did something more useful in the past, but right now it's
only used to avoid throttling some overflow-causing animations.

It returns 0 everywhere except on Android (for some reason?), but in any
case it doesn't seem this would need to be a LookAndFeel integer, it
could just be a regular pref that we turn on for tests.

However the tests pass with this patch locally, so for now I'm not
adding a pref to replace it.

Differential Revision: https://phabricator.services.mozilla.com/D120871

dom/animation/KeyframeEffect.cpp

@@ -1539,15 +1539,11 @@ bool KeyframeEffect::CanThrottleOverflowChangesInScrollable(
     return true;
   }
 
-  bool hasIntersectionObservers = doc->HasIntersectionObservers();
-
   // If we know that the animation cannot cause overflow,
   // we can just disable flushes for this animation.
 
-  // If we don't show scrollbars and have no intersection observers, we don't
-  // care about overflow.
-  if (LookAndFeel::GetInt(LookAndFeel::IntID::ShowHideScrollbars) == 0 &&
-      !hasIntersectionObservers) {
+  // If we have no intersection observers, we don't care about overflow.
+  if (!doc->HasIntersectionObservers()) {
     return true;
   }
 
@@ -1555,12 +1551,6 @@ bool KeyframeEffect::CanThrottleOverflowChangesInScrollable(
     return true;
   }
 
-  // If we have any intersection observers, we unthrottle this transform
-  // animation periodically.
-  if (hasIntersectionObservers) {
-    return false;
-  }
-
   // If the nearest scrollable ancestor has overflow:hidden,
   // we don't care about overflow.
   nsIScrollableFrame* scrollable =

dom/animation/test/chrome/test_running_on_compositor.html

@@ -247,9 +247,6 @@ promise_test(async t => {
 // This is to test that we don't temporarily clear the flag when forcing
 // an unthrottled sample.
 promise_test(async t => {
-  // Needs scrollbars to cause overflow.
-  await SpecialPowers.pushPrefEnv({ set: [["ui.showHideScrollbars", 1]] })
-
   var div = addDiv(t, { style: 'animation: rotate 100s' });
   var animation = div.getAnimations()[0];
 

dom/animation/test/mozilla/test_restyles.html

@@ -12,7 +12,6 @@ SpecialPowers.pushPrefEnv(
     set: [
       ['layout.reflow.synthMouseMove', false],
       ['privacy.reduceTimerPrecision', false],
-      ["ui.showHideScrollbars", 1],
     ],
   },
   function() {

mobile/android/app/mobile.js

@@ -56,7 +56,6 @@ pref("toolkit.zoomManager.zoomValues", ".2,.3,.5,.67,.8,.9,1,1.1,1.2,1.33,1.5,1.
 pref("browser.viewport.defaultZoom", -1);
 
 // Show/Hide scrollbars when active/inactive
-pref("ui.showHideScrollbars", 1);
 pref("ui.useOverlayScrollbars", 1);
 pref("ui.scrollbarFadeBeginDelay", 450);
 pref("ui.scrollbarFadeDuration", 0);

widget/LookAndFeel.h

@@ -58,8 +58,6 @@ class LookAndFeel {
     UseOverlayScrollbars,
     // allow H and V overlay scrollbars to overlap?
     AllowOverlayScrollbarsOverlap,
-    // show/hide scrollbars based on activity
-    ShowHideScrollbars,
     // skip navigating to disabled menu item?
     SkipNavigatingDisabledMenuItem,
     // begin a drag if the mouse is moved further than the threshold while the

widget/headless/HeadlessLookAndFeelGTK.cpp

@@ -153,9 +153,6 @@ nsresult HeadlessLookAndFeel::NativeGetInt(IntID aID, int32_t& aResult) {
     case IntID::AllowOverlayScrollbarsOverlap:
       aResult = 0;
       break;
-    case IntID::ShowHideScrollbars:
-      aResult = 0;
-      break;
     case IntID::SkipNavigatingDisabledMenuItem:
       aResult = 1;
       break;

widget/nsXPLookAndFeel.cpp

@@ -117,7 +117,6 @@ static const char sIntPrefs[][42] = {
     "ui.menusCanOverlapOSBar",
     "ui.useOverlayScrollbars",
     "ui.allowOverlayScrollbarsOverlap",
-    "ui.showHideScrollbars",
     "ui.skipNavigatingDisabledMenuItem",
     "ui.dragThresholdX",
     "ui.dragThresholdY",