Bug 1863620 - Disable overlay scrollbar animations in more tests.

MANUAL PUSH: Orange fix-up CLOSED TREE

testing/profiles/unittest-required/user.js

@@ -218,6 +218,8 @@ user_pref("toolkit.telemetry.test.pref1", true);
 user_pref("toolkit.telemetry.test.pref2", false);
 // Disable the caret blinking so we get stable snapshot
 user_pref("ui.caretBlinkTime", -1);
+// Disable scrollbar animations.
+user_pref("ui.scrollbarFadeDuration", 0);
 user_pref("webextensions.tests", true);
 // Disable intermittent telemetry collection
 user_pref("toolkit.telemetry.initDelay", 99999999);

testing/profiles/web-platform/user.js

@@ -59,12 +59,14 @@ user_pref("media.block-autoplay-until-in-foreground", false);
 // Disable dark scrollbars as it can be semi-transparent that many reftests
 // don't expect.
 user_pref("widget.disable-dark-scrollbar", true);
+// Disable scrollbar animations.
+user_pref("ui.scrollbarFadeDuration", 0);
 // Don't enable paint suppression when the background is unknown. While paint
 // is suppressed, synthetic click events and co. go to the old page, which can
 // be confusing for tests that send click events before the first paint.
 user_pref("nglayout.initialpaint.unsuppress_with_no_background", true);
 user_pref("media.block-autoplay-until-in-foreground", false);
-// Force a light color scheme unless explicitly overriden by pref.
+// Force a light color scheme unless explicitly overridden by pref.
 user_pref("layout.css.prefers-color-scheme.content-override", 1);
 // Force OffscreenCanvas support
 user_pref("gfx.offscreencanvas.enabled", true);