Clip display list based on frame viewport

Instead of creating a display list for the entire page, only create one
for an area that expands around the viewport. On my machine this makes
incremental layout of http://timecube.com 50% faster.

components/script/page.rs

@@ -19,19 +19,21 @@ use layout_interface::{
 };
 use script_traits::{UntrustedNodeAddress, ScriptControlChan};
 
-use geom::{Point2D, Rect};
+use geom::{Point2D, Rect, Size2D};
 use js::rust::Cx;
 use servo_msg::compositor_msg::PerformingLayout;
 use servo_msg::compositor_msg::ScriptListener;
 use servo_msg::constellation_msg::{ConstellationChan, WindowSizeData};
 use servo_msg::constellation_msg::{PipelineId, SubpageId};
 use servo_net::resource_task::ResourceTask;
-use servo_util::geometry::Au;
+use servo_util::geometry::{Au, MAX_RECT};
+use servo_util::geometry;
 use servo_util::str::DOMString;
 use servo_util::smallvec::{SmallVec1, SmallVec};
 use std::cell::Cell;
 use std::comm::{channel, Receiver, Empty, Disconnected};
 use std::mem::replace;
+use std::num::abs;
 use std::rc::Rc;
 use url::Url;
 
@@ -98,6 +100,10 @@ pub struct Page {
 
     /// Number of unnecessary potential reflows that were skipped since the last reflow
     pub avoided_reflows: Cell<int>,
+
+    /// An enlarged rectangle around the page contents visible in the viewport, used
+    /// to prevent creating display list items for content that is far away from the viewport.
+    pub page_clip_rect: Cell<Rect<Au>>,
 }
 
 pub struct PageIterator {
@@ -164,6 +170,7 @@ impl Page {
             damaged: Cell::new(false),
             pending_reflows: Cell::new(0),
             avoided_reflows: Cell::new(0),
+            page_clip_rect: Cell::new(MAX_RECT),
         }
     }
 
@@ -236,6 +243,49 @@ impl Page {
         }
         None
     }
+
+    fn should_move_clip_rect(&self, clip_rect: Rect<Au>, new_viewport: Rect<f32>) -> bool{
+        let clip_rect = Rect(Point2D(geometry::to_frac_px(clip_rect.origin.x) as f32,
+                                        geometry::to_frac_px(clip_rect.origin.y) as f32),
+                                Size2D(geometry::to_frac_px(clip_rect.size.width) as f32,
+                                       geometry::to_frac_px(clip_rect.size.height) as f32));
+
+        // We only need to move the clip rect if the viewport is getting near the edge of
+        // our preexisting clip rect. We use half of the size of the viewport as a heuristic
+        // for "close."
+        static VIEWPORT_SCROLL_MARGIN_SIZE: f32 = 0.5;
+        let viewport_scroll_margin = new_viewport.size * VIEWPORT_SCROLL_MARGIN_SIZE;
+
+        abs(clip_rect.origin.x - new_viewport.origin.x) <= viewport_scroll_margin.width ||
+        abs(clip_rect.max_x() - new_viewport.max_x()) <= viewport_scroll_margin.width ||
+        abs(clip_rect.origin.y - new_viewport.origin.y) <= viewport_scroll_margin.height ||
+        abs(clip_rect.max_y() - new_viewport.max_y()) <= viewport_scroll_margin.height
+    }
+
+    pub fn set_page_clip_rect_with_new_viewport(&self, viewport: Rect<f32>) -> bool {
+        // We use a clipping rectangle that is five times the size of the of the viewport,
+        // so that we don't collect display list items for areas too far outside the viewport,
+        // but also don't trigger reflows every time the viewport changes.
+        static VIEWPORT_EXPANSION: f32 = 2.0; // 2 lengths on each side plus original length is 5 total.
+        let proposed_clip_rect = geometry::f32_rect_to_au_rect(
+            viewport.inflate(viewport.size.width * VIEWPORT_EXPANSION,
+            viewport.size.height * VIEWPORT_EXPANSION));
+        let clip_rect = self.page_clip_rect.get();
+        if proposed_clip_rect == clip_rect {
+            return false;
+        }
+
+        let had_clip_rect = clip_rect != MAX_RECT;
+        if had_clip_rect && !self.should_move_clip_rect(clip_rect, viewport) {
+            return false;
+        }
+
+        self.page_clip_rect.set(proposed_clip_rect);
+
+        // If we didn't have a clip rect, the previous display doesn't need rebuilding
+        // because it was built for infinite clip (MAX_RECT).
+        had_clip_rect
+    }
 }
 
 impl Iterator<Rc<Page>> for PageIterator {
@@ -375,6 +425,7 @@ impl Page {
                     script_join_chan: join_chan,
                     id: last_reflow_id.get(),
                     query_type: query_type,
+                    page_clip_rect: self.page_clip_rect.get(),
                 };
 
                 let LayoutChan(ref chan) = self.layout_chan;