1 files changed, 21 insertions, 8 deletions
diff --git a/components/script/dom/blob.rs b/components/script/dom/blob.rs
index 27aa382c3fc..c5c5c480707 100644
--- a/components/script/dom/blob.rs
+++ b/components/script/dom/blob.rs
@@ -7,7 +7,7 @@ use std::ptr;
 use std::rc::Rc;
 
 use base::id::{BlobId, BlobIndex};
-use constellation_traits::BlobImpl;
+use constellation_traits::{BlobData, BlobImpl};
 use dom_struct::dom_struct;
 use encoding_rs::UTF_8;
 use js::jsapi::JSObject;
@@ -33,7 +33,7 @@ use crate::dom::readablestream::ReadableStream;
 use crate::realms::{AlreadyInRealm, InRealm};
 use crate::script_runtime::CanGc;
 
-// https://w3c.github.io/FileAPI/#blob
+/// <https://w3c.github.io/FileAPI/#dfn-Blob>
 #[dom_struct]
 pub(crate) struct Blob {
     reflector_: Reflector,
@@ -198,7 +198,7 @@ impl BlobMethods<crate::DomTypeHolder> for Blob {
         self.get_stream(can_gc)
     }
 
-    // https://w3c.github.io/FileAPI/#slice-method-algo
+    /// <https://w3c.github.io/FileAPI/#slice-method-algo>
     fn Slice(
         &self,
         start: Option<i64>,
@@ -206,11 +206,24 @@ impl BlobMethods<crate::DomTypeHolder> for Blob {
         content_type: Option<DOMString>,
         can_gc: CanGc,
     ) -> DomRoot<Blob> {
-        let type_string =
-            normalize_type_string(content_type.unwrap_or(DOMString::from("")).as_ref());
-        let rel_pos = RelativePos::from_opts(start, end);
-        let blob_impl = BlobImpl::new_sliced(rel_pos, self.blob_id, type_string);
-        Blob::new(&self.global(), blob_impl, can_gc)
+        let global = self.global();
+        let type_string = normalize_type_string(&content_type.unwrap_or_default());
+
+        // If our parent is already a sliced blob then we reference the data from the grandparent instead,
+        // to keep the blob ancestry chain short.
+        let (parent, range) = match *global.get_blob_data(&self.blob_id) {
+            BlobData::Sliced(grandparent, parent_range) => {
+                let range = RelativePos {
+                    start: parent_range.start + start.unwrap_or_default(),
+                    end: end.map(|end| end + parent_range.start).or(parent_range.end),
+                };
+                (grandparent, range)
+            },
+            _ => (self.blob_id, RelativePos::from_opts(start, end)),
+        };
+
+        let blob_impl = BlobImpl::new_sliced(range, parent, type_string);
+        Blob::new(&global, blob_impl, can_gc)
     }
 
     // https://w3c.github.io/FileAPI/#text-method-algo