Properly handle stacking context collection for truncated fragments

Before we did not properly descend intro truncated fragments when
collecting stacking contexts. This change makes sure that we descend
properly

Fixes #18254.
Fixes #17072.

components/layout/display_list_builder.rs

@@ -436,6 +436,10 @@ pub enum IdType {
 }
 
 pub trait FragmentDisplayListBuilding {
+    fn collect_stacking_contexts_for_blocklike_fragment(&mut self,
+                                                        state: &mut StackingContextCollectionState)
+                                                        -> bool;
+
     /// Adds the display items necessary to paint the background of this fragment to the display
     /// list if necessary.
     fn build_display_list_for_background_if_applicable(&self,
@@ -907,6 +911,33 @@ fn convert_ellipse_size_keyword(keyword: ShapeExtent,
 }
 
 impl FragmentDisplayListBuilding for Fragment {
+    fn collect_stacking_contexts_for_blocklike_fragment(&mut self,
+                                                        state: &mut StackingContextCollectionState)
+                                                        -> bool {
+        match self.specific {
+            SpecificFragmentInfo::InlineBlock(ref mut block_flow) => {
+                let block_flow = FlowRef::deref_mut(&mut block_flow.flow_ref);
+                block_flow.collect_stacking_contexts(state);
+                true
+            }
+            SpecificFragmentInfo::InlineAbsoluteHypothetical(ref mut block_flow) => {
+                let block_flow = FlowRef::deref_mut(&mut block_flow.flow_ref);
+                block_flow.collect_stacking_contexts(state);
+                true
+            }
+            SpecificFragmentInfo::InlineAbsolute(ref mut block_flow) => {
+                let block_flow = FlowRef::deref_mut(&mut block_flow.flow_ref);
+                block_flow.collect_stacking_contexts(state);
+                true
+            }
+            // FIXME: In the future, if #15144 is fixed we can remove this case. See #18510.
+            SpecificFragmentInfo::TruncatedFragment(ref mut info) => {
+                info.full.collect_stacking_contexts_for_blocklike_fragment(state)
+            }
+            _ => false,
+        }
+    }
+
     fn build_display_list_for_background_if_applicable(&self,
                                                        state: &mut DisplayListBuildState,
                                                        style: &ComputedValues,
@@ -2899,34 +2930,24 @@ impl InlineFlowDisplayListBuilding for InlineFlow {
                 state.containing_block_clip_and_scroll_info = state.current_clip_and_scroll_info;
             }
 
-            match fragment.specific {
+            if !fragment.collect_stacking_contexts_for_blocklike_fragment(state) {
-                SpecificFragmentInfo::InlineBlock(ref mut block_flow) => {
+                if fragment.establishes_stacking_context() {
-                    let block_flow = FlowRef::deref_mut(&mut block_flow.flow_ref);
-                    block_flow.collect_stacking_contexts(state);
-                }
-                SpecificFragmentInfo::InlineAbsoluteHypothetical(ref mut block_flow) => {
-                    let block_flow = FlowRef::deref_mut(&mut block_flow.flow_ref);
-                    block_flow.collect_stacking_contexts(state);
-                }
-                SpecificFragmentInfo::InlineAbsolute(ref mut block_flow) => {
-                    let block_flow = FlowRef::deref_mut(&mut block_flow.flow_ref);
-                    block_flow.collect_stacking_contexts(state);
-                }
-                _ if fragment.establishes_stacking_context() => {
                     fragment.stacking_context_id = fragment.stacking_context_id();
 
                     let current_stacking_context_id = state.current_stacking_context_id;
-                    let stacking_context = fragment.create_stacking_context(fragment.stacking_context_id,
+                    let stacking_context =
-                                                                            &self.base,
+                        fragment.create_stacking_context(fragment.stacking_context_id,
-                                                                            ScrollPolicy::Scrollable,
+                                                         &self.base,
-                                                                            StackingContextType::Real,
+                                                         ScrollPolicy::Scrollable,
-                                                                            state.current_clip_and_scroll_info);
+                                                         StackingContextType::Real,
+                                                         state.current_clip_and_scroll_info);
 
-                    state.add_stacking_context(current_stacking_context_id,
+                    state.add_stacking_context(current_stacking_context_id, stacking_context);
-                                               stacking_context);
+                } else {
+                    fragment.stacking_context_id = state.current_stacking_context_id;
                 }
-                _ => fragment.stacking_context_id = state.current_stacking_context_id,
             }
+
             state.containing_block_clip_and_scroll_info = previous_cb_clip_scroll_info;
         }
 

tests/wpt/mozilla/meta/MANIFEST.json

@@ -6353,6 +6353,18 @@
      {}
     ]
    ],
+   "css/truncated_text_fragment_stacking_context.html": [
+    [
+     "/_mozilla/css/truncated_text_fragment_stacking_context.html",
+     [
+      [
+       "/_mozilla/css/truncated_text_fragment_stacking_context_ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
    "css/upper_id_attr.html": [
     [
      "/_mozilla/css/upper_id_attr.html",
@@ -10547,6 +10559,11 @@
      {}
     ]
    ],
+   "css/truncated_text_fragment_stacking_context_ref.html": [
+    [
+     {}
+    ]
+   ],
    "css/upper_id_attr_ref.html": [
     [
      {}
@@ -26893,6 +26910,14 @@
    "c62420cce19d1d6a7d8700275daaa5ecc10d14c7",
    "support"
   ],
+  "css/truncated_text_fragment_stacking_context.html": [
+   "453b01aec6ffd3c854fda854cf81250a332cd55c",
+   "reftest"
+  ],
+  "css/truncated_text_fragment_stacking_context_ref.html": [
+   "4cb902bb7b058748d1702b5033e8cec729431640",
+   "support"
+  ],
   "css/upper_id_attr.html": [
    "f9965c1c22379aba20ac08d03a5e0e86cb1d8b99",
    "reftest"

tests/wpt/mozilla/tests/css/truncated_text_fragment_stacking_context.html

@@ -0,0 +1,33 @@
+<!DOCTYPE html>
+<title>Truncated text and inline-blocks combined should not cause a panic</title>
+<link rel="match" href="truncated_text_fragment_stacking_context_ref.html" />
+<link rel="help" href="https://www.w3.org/TR/css-position-3/#sticky-pos" />
+<meta name="assert" content="This test checks that truncated text and inline-blocks don't cause a panic." />
+
+<!-- We are using transparent text here to avoid having to deal with subpixel text rendering issues. -->
+<style>
+  a {
+    float: left;
+    color: transparent;
+  }
+  p, em {
+    overflow:hidden
+  }
+
+  em {
+    display: inline-block;
+    text-overflow: ellipsis;
+    color: transparent;
+  }
+
+  div {
+    position: absolute
+  }
+</style>
+
+<div style="height: 100px; width: 100px; background: green;"></div>
+
+<div>
+  <a>LINK</a>
+  <p><em>TEXT</em></p>
+</div>

tests/wpt/mozilla/tests/css/truncated_text_fragment_stacking_context_ref.html

@@ -0,0 +1,3 @@
+<!DOCTYPE html>
+<title>Truncated text and inline-blocks combined should not cause a panic</title>
+<div style="width: 100px; height: 100px; background: green"></div>