layout: Keep track of whether we've deferred the painting of the document due to

a script query.

This will, rather unfortunately, mean that we might repaint two times if we've
deferred a paint, then get an out-of-band reflow. Still seemed better than not
suppressing paints at all.

Fixes #13131

components/script/dom/document.rs

@@ -221,6 +221,9 @@ pub struct Document {
     /// For each element that has had a state or attribute change since the last restyle,
     /// track the original condition of the element.
     modified_elements: DOMRefCell<HashMap<JS<Element>, ElementSnapshot>>,
+    /// This flag will be true if layout suppressed a reflow attempt that was
+    /// needed in order for the page to be painted.
+    needs_paint: Cell<bool>,
     /// http://w3c.github.io/touch-events/#dfn-active-touch-point
     active_touch_points: DOMRefCell<Vec<JS<Touch>>>,
     /// Navigation Timing properties:
@@ -376,6 +379,10 @@ impl Document {
         }
     }
 
+    pub fn needs_paint(&self) -> bool {
+        self.needs_paint.get()
+    }
+
     pub fn needs_reflow(&self) -> bool {
         // FIXME: This should check the dirty bit on the document,
         // not the document element. Needs some layout changes to make
@@ -384,7 +391,8 @@ impl Document {
             Some(root) => {
                 root.upcast::<Node>().is_dirty() ||
                 root.upcast::<Node>().has_dirty_descendants() ||
-                !self.modified_elements.borrow().is_empty()
+                !self.modified_elements.borrow().is_empty() ||
+                self.needs_paint()
             }
             None => false,
         }
@@ -1602,6 +1610,8 @@ pub enum DocumentSource {
 pub trait LayoutDocumentHelpers {
     unsafe fn is_html_document_for_layout(&self) -> bool;
     unsafe fn drain_modified_elements(&self) -> Vec<(LayoutJS<Element>, ElementSnapshot)>;
+    unsafe fn needs_paint_from_layout(&self);
+    unsafe fn will_paint(&self);
 }
 
 #[allow(unsafe_code)]
@@ -1618,6 +1628,16 @@ impl LayoutDocumentHelpers for LayoutJS<Document> {
         let result = elements.drain().map(|(k, v)| (k.to_layout(), v)).collect();
         result
     }
+
+    #[inline]
+    unsafe fn needs_paint_from_layout(&self) {
+        (*self.unsafe_get()).needs_paint.set(true)
+    }
+
+    #[inline]
+    unsafe fn will_paint(&self) {
+        (*self.unsafe_get()).needs_paint.set(false)
+    }
 }
 
 /// https://url.spec.whatwg.org/#network-scheme
@@ -1723,6 +1743,7 @@ impl Document {
             base_element: Default::default(),
             appropriate_template_contents_owner_document: Default::default(),
             modified_elements: DOMRefCell::new(HashMap::new()),
+            needs_paint: Cell::new(false),
             active_touch_points: DOMRefCell::new(Vec::new()),
             dom_loading: Cell::new(Default::default()),
             dom_interactive: Cell::new(Default::default()),

components/script/dom/window.rs

@@ -1200,9 +1200,11 @@ impl Window {
         if !for_display || self.Document().needs_reflow() {
             issued_reflow = self.force_reflow(goal, query_type, reason);
 
-            // If window_size is `None`, we don't reflow, so the document stays dirty.
-            // Otherwise, we shouldn't need a reflow immediately after a reflow.
+            // If window_size is `None`, we don't reflow, so the document stays
+            // dirty. Otherwise, we shouldn't need a reflow immediately after a
+            // reflow, except if we're waiting for a deferred paint.
             assert!(!self.Document().needs_reflow() ||
+                    (!for_display && self.Document().needs_paint()) ||
                     self.window_size.get().is_none() ||
                     self.suppress_reflow.get());
         } else {

components/script/layout_wrapper.rs

@@ -405,6 +405,14 @@ impl<'ld> TDocument for ServoLayoutDocument<'ld> {
         let elements =  unsafe { self.document.drain_modified_elements() };
         elements.into_iter().map(|(el, snapshot)| (ServoLayoutElement::from_layout_js(el), snapshot)).collect()
     }
+
+    fn needs_paint_from_layout(&self) {
+        unsafe { self.document.needs_paint_from_layout(); }
+    }
+
+    fn will_paint(&self) {
+        unsafe { self.document.will_paint(); }
+    }
 }
 
 impl<'ld> ServoLayoutDocument<'ld> {