layout: Implement the correct hypothetical box behavior for

absolutely-positioned elements declared with `display: inline`.

Although the computed `display` property of elements with `position:
absolute` is `block`, `position: absolute; display: inline` can still
behave differently from `position: absolute; display: block`. This is
because the hypothetical box for `position: absolute` can be at the
position it would have been if it had `display: inline`. CSS 2.1 §
10.3.7 describes this case in a parenthetical:

"The static-position containing block is the containing block of a
hypothetical box that would have been the first box of the element if
its specified 'position' value had been 'static' and its specified
'float' had been 'none'. (Note that due to the rules in section 9.7 this
hypothetical calculation might require also assuming a different
computed value for 'display'.)"

To handle this, I had to change both style computation and layout. For
the former, I added an internal property
`-servo-display-for-hypothetical-box`, which stores the `display` value
supplied by the author, before the computed value is calculated. Flow
construction now uses this value.

As for layout, implementing the proper behavior is tricky because the
position of an inline fragment in the inline direction cannot be
determined until height assignment, which is a parallelism hazard
because in parallel layout widths are computed before heights. However,
in this particular case we can avoid the parallelism hazard because the
inline direction of a hypothetical box only affects the layout if an
absolutely-positioned element is unconstrained in the inline direction.
Therefore, we can just lay out such absolutely-positioned elements with
a bogus inline position and fix it up once the true inline position of
the hypothetical box is computed. The name for this fix-up process is
"late computation of inline position" (and the corresponding fix-up for
the block position is called "late computation of block position").

This improves the header on /r/rust.

components/style/properties/mod.rs.mako

@@ -133,7 +133,7 @@ pub mod longhands {
 
     <%def name="longhand(name, no_super=False, derived_from=None, experimental=False)">
         <%self:raw_longhand name="${name}" derived_from="${derived_from}"
-                            experimental="${experimental}">
+                            experimental="${experimental}" no_super="${no_super}">
             ${caller.body()}
             % if derived_from is None:
                 pub fn parse_specified(_input: &[ComponentValue], _base_url: &Url)
@@ -235,7 +235,7 @@ pub mod longhands {
     ${single_keyword("border-top-style", values="none solid double dotted dashed hidden groove ridge inset outset")}
 
     % for side in ["right", "bottom", "left"]:
-        <%self:longhand name="border-${side}-style", no_super="True">
+        <%self:longhand name="border-${side}-style">
             pub use super::border_top_style::{get_initial_value, parse, to_computed_value};
             pub type SpecifiedValue = super::border_top_style::SpecifiedValue;
             pub mod computed_value {
@@ -327,6 +327,25 @@ pub mod longhands {
     ${single_keyword("float", "none left right")}
     ${single_keyword("clear", "none left right both")}
 
+    <%self:longhand name="-servo-display-for-hypothetical-box" derived_from="display" no_super="True">
+        pub use super::computed_as_specified as to_computed_value;
+        pub use super::display::{SpecifiedValue, get_initial_value};
+        pub use super::display::{parse};
+        use super::computed;
+        use super::display;
+
+        pub mod computed_value {
+            pub type T = super::SpecifiedValue;
+        }
+
+        #[inline]
+        pub fn derive_from_display(_: display::computed_value::T, context: &computed::Context)
+                                   -> computed_value::T {
+            context.display
+        }
+
+    </%self:longhand>
+
     ${new_style_struct("InheritedBox", is_inherited=True)}
 
     ${single_keyword("direction", "ltr rtl", experimental=True)}
@@ -1840,7 +1859,7 @@ fn cascade_with_cached_declarations(applicable_declarations: &[DeclarationBlock]
         % if style_struct.inherited:
             let mut style_${style_struct.ident} = parent_style.${style_struct.ident}.clone();
         % else:
-            let style_${style_struct.ident} = cached_style.${style_struct.ident}.clone();
+            let mut style_${style_struct.ident} = cached_style.${style_struct.ident}.clone();
         % endif
     % endfor