layout: Implement z-index.

components/gfx/display_list/mod.rs

@@ -177,9 +177,6 @@ struct StackingContext {
     /// All other content.
     pub content: DisplayList,
     /// Positioned descendant stacking contexts, along with their `z-index` levels.
-    ///
-    /// TODO(pcwalton): `z-index` should be the actual CSS property value in order to handle
-    /// `auto`, not just an integer.
     pub positioned_descendants: Vec<(i32, DisplayList)>,
 }
 
@@ -383,7 +380,10 @@ impl DisplayList {
         // Steps 1 and 2: Borders and background for the root.
         result.push_all_move(background_and_borders);
 
-        // TODO(pcwalton): Sort positioned children according to z-index.
+        // Sort positioned children according to z-index.
+        positioned_descendants.sort_by(|&(z_index_a, _), &(z_index_b, _)| {
+            z_index_a.cmp(&z_index_b)
+        });
 
         // Step 3: Positioned descendants with negative z-indices.
         for &(ref mut z_index, ref mut list) in positioned_descendants.iter_mut() {

components/layout/block.rs

@@ -1239,11 +1239,10 @@ impl BlockFlow {
         if !self.base.absolute_position_info.layers_needed_for_positioned_flows &&
                 !self.base.flags.needs_layer() {
             // We didn't need a layer.
-            //
-            // TODO(#781, pcwalton): `z-index`.
-            self.base.display_list =
-                mem::replace(&mut self.base.display_list,
-                             DisplayList::new()).flatten(PositionedDescendantStackingLevel(0));
+            let z_index = self.fragment.style().get_box().z_index.number_or_zero();
+            let level = PositionedDescendantStackingLevel(z_index);
+            self.base.display_list = mem::replace(&mut self.base.display_list,
+                                                  DisplayList::new()).flatten(level);
             return
         }
 

components/layout/construct.rs

@@ -330,12 +330,17 @@ impl<'a> FlowConstructor<'a> {
 
         {
             let inline_flow = inline_flow_ref.get_mut().as_inline();
+
+            // We must scan for runs before computing minimum ascent and descent because scanning
+            // for runs might collapse so much whitespace away that only hypothetical fragments
+            // remain. In that case the inline flow will compute its ascent and descent to be zero.
+            TextRunScanner::new().scan_for_runs(self.layout_context.font_context(), inline_flow);
+
             let (ascent, descent) =
                 inline_flow.compute_minimum_ascent_and_descent(self.layout_context.font_context(),
                                                                &**node.style());
             inline_flow.minimum_block_size_above_baseline = ascent;
             inline_flow.minimum_depth_below_baseline = descent;
-            TextRunScanner::new().scan_for_runs(self.layout_context.font_context(), inline_flow);
         }
 
         inline_flow_ref.finish(self.layout_context);

components/layout/fragment.rs

@@ -1701,13 +1701,13 @@ impl Fragment {
                                                                 &font_style);
                 InlineMetrics::from_block_height(&font_metrics, block_flow.base.position.size.block)
             }
-            InlineAbsoluteHypotheticalFragment(ref info) => {
-                // See CSS 2.1 § 10.8.1.
-                let block_flow = info.flow_ref.get().as_immutable_block();
-                let font_style = text::computed_style_to_font_style(&*self.style);
-                let font_metrics = text::font_metrics_for_style(layout_context.font_context(),
-                                                                &font_style);
-                InlineMetrics::from_block_height(&font_metrics, block_flow.base.position.size.block)
+            InlineAbsoluteHypotheticalFragment(_) => {
+                // Hypothetical boxes take up no space.
+                InlineMetrics {
+                    block_size_above_baseline: Au(0),
+                    depth_below_baseline: Au(0),
+                    ascent: Au(0),
+                }
             }
             _ => {
                 InlineMetrics {
@@ -1719,6 +1719,14 @@ impl Fragment {
         }
     }
 
+    /// Returns true if this fragment is a hypothetical box. See CSS 2.1 § 10.3.7.
+    pub fn is_hypothetical(&self) -> bool {
+        match self.specific {
+            InlineAbsoluteHypotheticalFragment(_) => true,
+            _ => false,
+        }
+    }
+
     /// Returns true if this fragment can merge with another adjacent fragment or false otherwise.
     pub fn can_merge_with_fragment(&self, other: &Fragment) -> bool {
         match (&self.specific, &other.specific) {

components/layout/inline.rs

@@ -943,7 +943,14 @@ impl InlineFlow {
     /// `style` is the style of the block.
     pub fn compute_minimum_ascent_and_descent(&self,
                                               font_context: &mut FontContext,
-                                              style: &ComputedValues) -> (Au, Au) {
+                                              style: &ComputedValues)
+                                              -> (Au, Au) {
+        // As a special case, if this flow contains only hypothetical fragments, then the entire
+        // flow is hypothetical and takes up no space. See CSS 2.1 § 10.3.7.
+        if self.fragments.fragments.iter().all(|fragment| fragment.is_hypothetical()) {
+            return (Au(0), Au(0))
+        }
+
         let font_style = text::computed_style_to_font_style(style);
         let font_metrics = text::font_metrics_for_style(font_context, &font_style);
         let line_height = text::line_height_from_style(style, &font_metrics);

components/layout/layout_task.rs

@@ -800,6 +800,9 @@ impl LayoutTask {
         if self.opts.trace_layout {
             layout_debug::begin_trace(layout_root.clone());
         }
+        if self.opts.dump_flow_tree {
+            layout_root.get_mut().dump();
+        }
 
         // Propagate damage.
         profile(time::LayoutDamagePropagateCategory, Some((&data.url, data.iframe, self.first_reflow.get())),

components/style/properties/mod.rs.mako

@@ -345,6 +345,41 @@ pub mod longhands {
 
     </%self:longhand>
 
+    <%self:single_component_value name="z-index">
+        pub use super::computed_as_specified as to_computed_value;
+        pub type SpecifiedValue = computed_value::T;
+        pub mod computed_value {
+            #[deriving(PartialEq, Clone)]
+            pub enum T {
+                Auto,
+                Number(i32),
+            }
+
+            impl T {
+                pub fn number_or_zero(self) -> i32 {
+                    match self {
+                        Auto => 0,
+                        Number(value) => value,
+                    }
+                }
+            }
+        }
+        #[inline]
+        pub fn get_initial_value() -> computed_value::T {
+            Auto
+        }
+        fn from_component_value(input: &ComponentValue, _: &Url) -> Result<SpecifiedValue,()> {
+            match *input {
+                Ident(ref keyword) if keyword.as_slice().eq_ignore_ascii_case("auto") => Ok(Auto),
+                ast::Number(ast::NumericValue {
+                    int_value: Some(value),
+                    ..
+                }) => Ok(Number(value as i32)),
+                _ => Err(())
+            }
+        }
+    </%self:single_component_value>
+
     ${new_style_struct("InheritedBox", is_inherited=True)}
 
     ${single_keyword("direction", "ltr rtl", experimental=True)}

components/util/opts.rs

@@ -94,6 +94,9 @@ pub struct Opts {
 
     /// An optional string allowing the user agent to be set for testing.
     pub user_agent: Option<String>,
+
+    /// Dumps the flow tree after a layout.
+    pub dump_flow_tree: bool,
 }
 
 fn print_usage(app: &str, opts: &[getopts::OptGroup]) {
@@ -131,6 +134,7 @@ pub fn from_cmdline_args(args: &[String]) -> Option<Opts> {
         getopts::optflagopt("", "devtools", "Start remote devtools server on port", "6000"),
         getopts::optopt("", "resolution", "Set window resolution.", "800x600"),
         getopts::optopt("u", "user-agent", "Set custom user agent string", "NCSA Mosaic/1.0 (X11;SunOS 4.1.4 sun4m)"),
+        getopts::optflag("", "dump-flow-tree", "Dump the flow (render) tree during each layout."),
         getopts::optflag("h", "help", "Print this message")
     );
 
@@ -248,6 +252,7 @@ pub fn from_cmdline_args(args: &[String]) -> Option<Opts> {
         devtools_port: devtools_port,
         initial_window_size: initial_window_size,
         user_agent: opt_match.opt_str("u"),
+        dump_flow_tree: opt_match.opt_present("dump-flow-tree"),
     })
 }
 

ports/cef/core.rs

@@ -71,6 +71,7 @@ pub extern "C" fn cef_run_message_loop() {
         devtools_port: None,
         initial_window_size: TypedSize2D(800, 600),
         user_agent: None,
+        dump_flow_tree: false,
     };
     native::start(0, 0 as *const *const u8, proc() {
        servo::run(opts);