layout: Convert layout internal display to inline for replaced elements (#31133)

Replaced elements should never be able to have a layout internal
display, according to the specification. This change makes it so that
the used value of replaced element's display is always inline, as the
specification says.

components/layout_2020/dom_traversal.rs

@@ -172,6 +172,7 @@ fn traverse_element<'dom, Node>(
         },
         Display::GeneratingBox(display) => {
             let contents = replaced.map_or(Contents::OfElement, Contents::Replaced);
+            let display = display.used_value_for_contents(&contents);
             let box_slot = element.element_box_slot();
             let info = NodeAndStyleInfo::new(element, style);
             handler.handle_element(&info, display, contents, box_slot);

components/layout_2020/style_ext.rs

@@ -21,6 +21,7 @@ use style::values::specified::{box_ as stylo, Overflow};
 use style::Zero;
 use webrender_api as wr;
 
+use crate::dom_traversal::Contents;
 use crate::geom::{
     LengthOrAuto, LengthPercentageOrAuto, LogicalSides, LogicalVec2, PhysicalSides, PhysicalSize,
 };
@@ -52,6 +53,23 @@ impl DisplayGeneratingBox {
             },
         }
     }
+
+    pub(crate) fn used_value_for_contents(&self, contents: &Contents) -> Self {
+        // From <https://www.w3.org/TR/css-display-3/#layout-specific-display>:
+        // > When the display property of a replaced element computes to one of
+        // > the layout-internal values, it is handled as having a used value of
+        // > inline.
+        if matches!(self, Self::LayoutInternal(_)) && contents.is_replaced() {
+            Self::OutsideInside {
+                outside: DisplayOutside::Inline,
+                inside: DisplayInside::Flow {
+                    is_list_item: false,
+                },
+            }
+        } else {
+            *self
+        }
+    }
 }
 
 #[derive(Clone, Copy, Eq, PartialEq)]

components/layout_2020/table/construct.rs

@@ -679,7 +679,7 @@ where
                             )
                         },
                         Err(_replaced) => {
-                            panic!("We don't handle this yet.");
+                            unreachable!("Replaced should not have a LayoutInternal display type.");
                         },
                     };