Bug 1738663 - Add a pref to expose all shorthands in the computed style. r=layout-reviewers,mats

Make it always false for now, as we still need a solution for
layout-dependent shorthands like margin, padding, border, etc.

Differential Revision: https://phabricator.services.mozilla.com/D130039

layout/style/CSSPropFlags.h

@@ -50,6 +50,10 @@ enum class CSSPropFlags : uint8_t {
 
   // Whether this is a logical property.
   IsLogical = 1 << 6,
+
+  // Whether this shorthand property is unconditionally exposed in
+  // getComputedStyle.
+  ShorthandUnconditionallyExposedOnGetCS = 1 << 7,
 };
 
 MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(CSSPropFlags)

layout/style/ServoCSSPropList.mako.py

@@ -110,11 +110,9 @@ def serialized_by_servo(prop):
 def exposed_on_getcs(prop):
     if "Style" not in prop.rule_types_allowed_names():
         return False
-    if prop.type() == "longhand":
+    if is_internal(prop):
-        return not is_internal(prop)
+        return False
-    # TODO: bug 137688 / https://github.com/w3c/csswg-drafts/issues/2529
+    return True
-    if prop.type() == "shorthand":
-        return "SHORTHAND_IN_GETCS" in prop.flags
 
 def rules(prop):
     return ", ".join('"{}"'.format(rule) for rule in prop.rule_types_allowed_names())
@@ -133,6 +131,8 @@ def flags(prop):
         result.append("CanAnimateOnCompositor")
     if exposed_on_getcs(prop):
         result.append("ExposedOnGetCS")
+        if prop.type() == "shorthand" and "SHORTHAND_IN_GETCS" in prop.flags:
+            result.append("ShorthandUnconditionallyExposedOnGetCS")
         if serialized_by_servo(prop):
             result.append("SerializedByServo")
     if prop.type() == "longhand" and prop.logical:

layout/style/nsComputedDOMStyle.cpp

@@ -212,7 +212,15 @@ struct ComputedStyleMap {
     ComputeMethod mGetter;
 
     bool IsEnabled() const {
-      return nsCSSProps::IsEnabled(mProperty, CSSEnabledState::ForAllContent);
+      if (!nsCSSProps::IsEnabled(mProperty, CSSEnabledState::ForAllContent)) {
+        return false;
+      }
+      if (nsCSSProps::IsShorthand(mProperty) &&
+          !StaticPrefs::layout_css_computed_style_shorthands()) {
+        return nsCSSProps::PropHasFlags(
+            mProperty, CSSPropFlags::ShorthandUnconditionallyExposedOnGetCS);
+      }
+      return true;
     }
   };
 
@@ -2506,6 +2514,10 @@ void nsComputedDOMStyle::RegisterPrefChangeCallbacks() {
       prefs.InsertElementSorted(p->mPref);
     }
   }
+
+  prefs.AppendElement(
+      StaticPrefs::GetPrefName_layout_css_computed_style_shorthands());
+
   prefs.AppendElement(nullptr);
 
   MOZ_ASSERT(!gCallbackPrefs);

modules/libpref/init/StaticPrefList.yaml

@@ -6903,6 +6903,12 @@
   value: true
   mirror: always
 
+# Whether we should expose all shorthands in getComputedStyle().
+- name: layout.css.computed-style.shorthands
+  type: bool
+  value: false
+  mirror: always
+
 # Are implicit tracks in computed grid templates serialized?
 - name: layout.css.serialize-grid-implicit-tracks
   type: RelaxedAtomicBool