style: Dishonor display: -moz-box if -webkit-box was specified before.

Bug: 1407701

components/style/properties/declaration_block.rs

@@ -500,6 +500,25 @@ impl PropertyDeclarationBlock {
                             return true;
                         }
                         DeclarationSource::Parsing => {
+                            // As a compatibility hack, specially on Android,
+                            // don't allow to override a prefixed webkit display
+                            // value with an unprefixed version from parsing
+                            // code.
+                            //
+                            // TODO(emilio): Unship.
+                            if let PropertyDeclaration::Display(old_display) = *slot {
+                                use properties::longhands::display::computed_value::T as display;
+
+                                let new_display = match declaration {
+                                    PropertyDeclaration::Display(new_display) => new_display,
+                                    _ => unreachable!("How could the declaration id be the same?"),
+                                };
+
+                                if display::should_ignore_parsed_value(old_display, new_display) {
+                                    return false;
+                                }
+                            }
+
                             // NOTE(emilio): We could avoid this and just override for
                             // properties not affected by logical props, but it's not
                             // clear it's worth it given the `definitely_new` check.

components/style/properties/longhand/box.mako.rs

@@ -68,6 +68,33 @@
                 )
             }
 
+            /// Whether `new_display` should be ignored, given a previous
+            /// `old_display` value.
+            ///
+            /// This is used to ignore `display: -moz-box` declarations after an
+            /// equivalent `display: -webkit-box` declaration, since the former
+            /// has a vastly different meaning. See bug 1107378 and bug 1407701.
+            ///
+            /// FIXME(emilio): This is a pretty decent hack, we should try to
+            /// remove it.
+            pub fn should_ignore_parsed_value(
+                _old_display: Self,
+                _new_display: Self,
+            ) -> bool {
+                #[cfg(feature = "gecko")]
+                {
+                    match (_old_display, _new_display) {
+                        (T::_webkit_box, T::_moz_box) |
+                        (T::_webkit_inline_box, T::_moz_inline_box) => {
+                            return true;
+                        }
+                        _ => {},
+                    }
+                }
+
+                return false;
+            }
+
             /// Returns whether this "display" value is one of the types for
             /// ruby.
             #[cfg(feature = "gecko")]