style: Implement page-break-{before,after} as legacy shorthands for {before,after}.

This is all the style-system work needed for this.

This implements the concept of legacy shorthands, teaches tests to understand
it, and adds a few more tests for these properties in particular.

The WPT even caught a few WebKit / Blink bugs:

  https://bugs.chromium.org/p/chromium/issues/detail?id=906336
  https://bugs.webkit.org/show_bug.cgi?id=191803

This doesn't change the layout behavior for page-break-before: always, since
it'd stop breaking in multicol and such. Similarly, break-before / break-after:
column and page still behave the same, I'll file followups for those given
comment 22.

Differential Revision: https://phabricator.services.mozilla.com/D12211

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

@@ -438,22 +438,22 @@ ${helpers.single_keyword(
 )}
 
 ${helpers.predefined_type(
-    "page-break-after",
+    "break-after",
     "BreakBetween",
     "computed::BreakBetween::Auto",
     needs_context=False,
     products="gecko",
-    spec="https://drafts.csswg.org/css2/page.html#propdef-page-break-after",
+    spec="https://drafts.csswg.org/css-break/#propdef-break-after",
     animation_value_type="discrete",
 )}
 
 ${helpers.predefined_type(
-    "page-break-before",
+    "break-before",
     "BreakBetween",
     "computed::BreakBetween::Auto",
     needs_context=False,
     products="gecko",
-    spec="https://drafts.csswg.org/css2/page.html#propdef-page-break-before",
+    spec="https://drafts.csswg.org/css-break/#propdef-break-before",
     animation_value_type="discrete",
 )}
 

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

@@ -447,3 +447,49 @@ macro_rules! try_parse_one {
         }
     }
 </%helpers:shorthand>
+
+<%helpers:shorthand
+    name="page-break-before"
+    flags="SHORTHAND_IN_GETCS IS_LEGACY_SHORTHAND"
+    sub_properties="break-before"
+    spec="https://drafts.csswg.org/css2/page.html#propdef-page-break-before"
+>
+    pub fn parse_value<'i>(
+        _: &ParserContext,
+        input: &mut Parser<'i, '_>,
+    ) -> Result<Longhands, ParseError<'i>> {
+        use crate::values::specified::box_::BreakBetween;
+        Ok(expanded! {
+            break_before: BreakBetween::parse_legacy(input)?,
+        })
+    }
+
+    impl<'a> ToCss for LonghandsToSerialize<'a> {
+        fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result where W: fmt::Write {
+            self.break_before.to_css_legacy(dest)
+        }
+    }
+</%helpers:shorthand>
+
+<%helpers:shorthand
+    name="page-break-after"
+    flags="SHORTHAND_IN_GETCS IS_LEGACY_SHORTHAND"
+    sub_properties="break-after"
+    spec="https://drafts.csswg.org/css2/page.html#propdef-page-break-after"
+>
+    pub fn parse_value<'i>(
+        _: &ParserContext,
+        input: &mut Parser<'i, '_>,
+    ) -> Result<Longhands, ParseError<'i>> {
+        use crate::values::specified::box_::BreakBetween;
+        Ok(expanded! {
+            break_after: BreakBetween::parse_legacy(input)?,
+        })
+    }
+
+    impl<'a> ToCss for LonghandsToSerialize<'a> {
+        fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result where W: fmt::Write {
+            self.break_after.to_css_legacy(dest)
+        }
+    }
+</%helpers:shorthand>

components/style/values/specified/box.rs

@@ -1286,13 +1286,60 @@ pub enum Appearance {
 )]
 #[repr(u8)]
 pub enum BreakBetween {
-    Auto,
     Always,
+    Auto,
+    Page,
     Avoid,
     Left,
     Right,
 }
 
+impl BreakBetween {
+    /// Parse a legacy break-between value for `page-break-*`.
+    ///
+    /// See https://drafts.csswg.org/css-break/#page-break-properties.
+    #[inline]
+    pub fn parse_legacy<'i>(
+        input: &mut Parser<'i, '_>,
+    ) -> Result<Self, ParseError<'i>> {
+        let location = input.current_source_location();
+        let ident = input.expect_ident()?;
+        let break_value = match BreakBetween::from_ident(ident) {
+            Ok(v) => v,
+            Err(()) => return Err(location.new_custom_error(
+                SelectorParseErrorKind::UnexpectedIdent(ident.clone())
+            )),
+        };
+        match break_value {
+            BreakBetween::Always => Ok(BreakBetween::Page),
+            BreakBetween::Auto |
+            BreakBetween::Avoid |
+            BreakBetween::Left |
+            BreakBetween::Right => Ok(break_value),
+            BreakBetween::Page => {
+                Err(location.new_custom_error(SelectorParseErrorKind::UnexpectedIdent(ident.clone())))
+            }
+        }
+    }
+
+    /// Serialize a legacy break-between value for `page-break-*`.
+    ///
+    /// See https://drafts.csswg.org/css-break/#page-break-properties.
+    pub fn to_css_legacy<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        match *self {
+            BreakBetween::Auto |
+            BreakBetween::Avoid |
+            BreakBetween::Left |
+            BreakBetween::Right => self.to_css(dest),
+            BreakBetween::Page => dest.write_str("always"),
+            BreakBetween::Always => Ok(()),
+        }
+    }
+}
+
 /// A kind of break within a box.
 ///
 /// https://drafts.csswg.org/css-break/#break-within