Auto merge of #16407 - jdm:style_panic, r=emilio

Allow loading multiple stylesheets for the same link element.

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #16399
- [x] There are tests for these changes

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/16407)
<!-- Reviewable:end -->

components/script/dom/htmllinkelement.rs

@@ -24,6 +24,7 @@ use dom::virtualmethods::VirtualMethods;
 use dom_struct::dom_struct;
 use html5ever_atoms::LocalName;
 use net_traits::ReferrerPolicy;
+use script_layout_interface::message::Msg;
 use script_traits::{MozBrowserEvent, ScriptMsg as ConstellationMsg};
 use std::ascii::AsciiExt;
 use std::borrow::ToOwned;
@@ -97,8 +98,9 @@ impl HTMLLinkElement {
     }
 
     pub fn set_stylesheet(&self, s: Arc<Stylesheet>) {
-        assert!(self.stylesheet.borrow().is_none()); // Useful for catching timing issues.
-        *self.stylesheet.borrow_mut() = Some(s);
+        *self.stylesheet.borrow_mut() = Some(s.clone());
+        window_from_node(self).layout_chan().send(Msg::AddStylesheet(s)).unwrap();
+        document_from_node(self).invalidate_stylesheets();
     }
 
     pub fn get_stylesheet(&self) -> Option<Arc<Stylesheet>> {

components/script/stylesheet_loader.rs

@@ -22,7 +22,6 @@ use ipc_channel::router::ROUTER;
 use net_traits::{FetchResponseListener, FetchMetadata, FilteredMetadata, Metadata, NetworkError, ReferrerPolicy};
 use net_traits::request::{CorsSettings, CredentialsMode, Destination, RequestInit, RequestMode, Type as RequestType};
 use network_listener::{NetworkListener, PreInvoke};
-use script_layout_interface::message::Msg;
 use servo_url::ServoUrl;
 use std::mem;
 use std::sync::{Arc, Mutex};
@@ -151,9 +150,7 @@ impl FetchResponseListener for StylesheetContext {
                             sheet.set_disabled(true);
                         }
 
-                        link.set_stylesheet(sheet.clone());
-
-                        win.layout_chan().send(Msg::AddStylesheet(sheet)).unwrap();
+                        link.set_stylesheet(sheet);
                     }
                 }
                 StylesheetContextSource::Import(ref stylesheet) => {