Bug 1564597 - Ignore pref-controlled properties in cached scrollbar style assertions. r=emilio

Now if you add a new inherited, pref-controlled property, you must
declare whether it can have an effect on scrollbar styles.  If no,
then the property will be skipped in the assertions that check
whether our cached styles are equal to those we would compute.

Differential Revision: https://phabricator.services.mozilla.com/D37507

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

servo/components/style/properties/data.py

@@ -175,6 +175,7 @@ class Longhand(object):
                  predefined_type=None, servo_pref=None, gecko_pref=None,
                  enabled_in="content", need_index=False,
                  gecko_ffi_name=None,
+                 has_effect_on_gecko_scrollbars=None,
                  allowed_in_keyframe_block=True, cast_type='u8',
                  logical=False, logical_group=None, alias=None, extra_prefixes=None, boxed=False,
                  flags=None, allowed_in_page_rule=False, allow_quirks="No",
@@ -192,6 +193,14 @@ class Longhand(object):
         self.style_struct = style_struct
         self.servo_pref = servo_pref
         self.gecko_pref = gecko_pref
+        self.has_effect_on_gecko_scrollbars = has_effect_on_gecko_scrollbars
+        assert (
+            has_effect_on_gecko_scrollbars in [None, False, True] and
+            not style_struct.inherited or
+            (gecko_pref is None) == (has_effect_on_gecko_scrollbars is None)), (
+            "Property " + name + ": has_effect_on_gecko_scrollbars must be " +
+            "specified, and must have a value of True or False, iff a " +
+            "property is inherited and is behind a Gecko pref")
         # For enabled_in, the setup is as follows:
         # It needs to be one of the four values: ["", "ua", "chrome", "content"]
         #  * "chrome" implies "ua", and implies that they're explicitly

servo/components/style/properties/longhands/font.mako.rs

@@ -192,6 +192,7 @@ ${helpers.predefined_type(
     "FontVariationSettings",
     products="gecko",
     gecko_pref="layout.css.font-variations.enabled",
+    has_effect_on_gecko_scrollbars=False,
     initial_value="computed::FontVariationSettings::normal()",
     initial_specified_value="specified::FontVariationSettings::normal()",
     animation_value_type="ComputedValue",
@@ -216,6 +217,7 @@ ${helpers.single_keyword_system(
     "auto none",
     products="gecko",
     gecko_pref="layout.css.font-variations.enabled",
+    has_effect_on_gecko_scrollbars=False,
     gecko_ffi_name="mFont.opticalSizing",
     gecko_constant_prefix="NS_FONT_OPTICAL_SIZING",
     animation_value_type="discrete",
@@ -512,6 +514,7 @@ ${helpers.single_keyword(
     gecko_constant_prefix="NS_FONT_SMOOTHING",
     gecko_ffi_name="mFont.smoothing",
     gecko_pref="layout.css.osx-font-smoothing.enabled",
+    has_effect_on_gecko_scrollbars=False,
     products="gecko",
     spec="Nonstandard (https://developer.mozilla.org/en-US/docs/Web/CSS/font-smooth)",
     flags="APPLIES_TO_CUE APPLIES_TO_FIRST_LETTER APPLIES_TO_FIRST_LINE APPLIES_TO_PLACEHOLDER APPLIES_TO_MARKER",

servo/components/style/properties/longhands/inherited_text.mako.rs

@@ -103,7 +103,8 @@ ${helpers.predefined_type(
     gecko_enum_prefix="StyleTextJustify"
     animation_value_type="discrete"
     gecko_pref="layout.css.text-justify.enabled"
-    flags="APPLIES_TO_PLACEHOLDER",
+    has_effect_on_gecko_scrollbars="False"
+    flags="APPLIES_TO_PLACEHOLDER"
     spec="https://drafts.csswg.org/css-text/#propdef-text-justify"
     servo_restyle_damage="rebuild_and_reflow"
 >
@@ -383,6 +384,7 @@ ${helpers.predefined_type(
     products="gecko",
     animation_value_type="ComputedValue",
     gecko_pref="layout.css.text-underline-offset.enabled",
+    has_effect_on_gecko_scrollbars=False,
     spec="https://drafts.csswg.org/css-text-decor-4/#underline-offset",
 )}
 
@@ -395,5 +397,6 @@ ${helpers.predefined_type(
     needs_context=False,
     animation_value_type="discrete",
     gecko_pref="layout.css.text-decoration-skip-ink.enabled",
+    has_effect_on_gecko_scrollbars=False,
     spec="https://drafts.csswg.org/css-text-decor-4/#text-decoration-skip-ink-property",
 )}

servo/components/style/properties/longhands/inherited_ui.mako.rs

@@ -74,6 +74,10 @@ ${helpers.predefined_type(
     "Default::default()",
     spec="https://drafts.csswg.org/css-scrollbars-1/#scrollbar-color",
     gecko_pref="layout.css.scrollbar-color.enabled",
+    # Surprisingly, yes the computed value of scrollbar-color has no effect on
+    # Gecko scrollbar elements, since the value only matters on the scrollable
+    # element itself.
+    has_effect_on_gecko_scrollbars=False,
     animation_value_type="ScrollbarColor",
     boxed=True,
     ignored_when_colors_disabled=True,

servo/components/style/properties/properties.mako.rs

@@ -812,6 +812,22 @@ impl LonghandIdSet {
         &IGNORED_WHEN_COLORS_DISABLED
     }
 
+    /// Returns the set of properties that are declared as having no effect on
+    /// Gecko <scrollbar> elements or their descendant scrollbar parts.
+    #[cfg(debug_assertions)]
+    #[cfg(feature = "gecko")]
+    #[inline]
+    pub fn has_no_effect_on_gecko_scrollbars() -> &'static Self {
+        // data.py asserts that has_no_effect_on_gecko_scrollbars is True or
+        // False for properties that are inherited and Gecko pref controlled,
+        // and is None for all other properties.
+        ${static_longhand_id_set(
+            "HAS_NO_EFFECT_ON_SCROLLBARS",
+            lambda p: p.has_effect_on_gecko_scrollbars is False
+        )}
+        &HAS_NO_EFFECT_ON_SCROLLBARS
+    }
+
     /// Iterate over the current longhand id set.
     pub fn iter(&self) -> LonghandIdSetIterator {
         LonghandIdSetIterator { longhands: self, cur: 0, }

servo/ports/geckolib/glue.rs

@@ -3799,6 +3799,24 @@ pub extern "C" fn Servo_ComputedValues_EqualForCachedAnonymousContentStyle(
     // anonymous content subtrees we cache style for.
     differing_properties.remove(LonghandId::XLang);
 
+    // Ignore any difference in pref-controlled, inherited properties.  These
+    // properties may or may not be set by the 'all' declaration in the
+    // minimal-xul.css rule, depending on whether the pref was enabled at the
+    // time the UA sheets were parsed.
+    //
+    // If you add a new pref-controlled, inherited property, it must be defined
+    // with `has_effect_on_gecko_scrollbars=False` to declare that
+    // different values of this property on a <scrollbar> element or its
+    // descendant scrollbar part elements should have no effect on their
+    // rendering and behavior.
+    //
+    // If you do need a pref-controlled, inherited property to have an effect
+    // on these elements, then you will need to add some checks to the
+    // nsIAnonymousContentCreator::CreateAnonymousContent implementations of
+    // ScrollFrameHelper and nsScrollbarFrame to clear the AnonymousContentKey
+    // if a non-initial value is used.
+    differing_properties.remove_all(&LonghandIdSet::has_no_effect_on_gecko_scrollbars());
+
     if !differing_properties.is_empty() {
         println_stderr!("Actual style:");
         dump_properties_and_rules(a, &differing_properties);