servoshell: Support runtime preference manipulation (#38159)

These changes add a custom servo:preferences URL that allows modifying
selected preferences at runtime. The goal of this work is to make it
easy to test pages while toggling experimental web platform features,
and support quickly changing the User-Agent header.

Testing: Manually verified that spacex.com loads correctly after
changing the user agent, and that https://polygon.io/ displays grid
elements correctly and no console errors with the experimental prefs
enabled.
Fixes: #35862

<img width="1136" height="880" alt="Screenshot 2025-07-18 at 1 06 23 AM"
src="https://github.com/user-attachments/assets/2d27c321-6ca0-43c3-a347-7bc4b55272df"
/>

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/script/dom/servointernals.rs

@@ -8,11 +8,13 @@ use constellation_traits::ScriptToConstellationMessage;
 use dom_struct::dom_struct;
 use js::rust::HandleObject;
 use profile_traits::mem::MemoryReportResult;
+use script_bindings::error::{Error, Fallible};
 use script_bindings::interfaces::ServoInternalsHelpers;
 use script_bindings::script_runtime::JSContext;
+use script_bindings::str::USVString;
+use servo_config::prefs::{self, PrefValue};
 
 use crate::dom::bindings::codegen::Bindings::ServoInternalsBinding::ServoInternalsMethods;
-use crate::dom::bindings::error::Error;
 use crate::dom::bindings::reflector::{DomGlobal, Reflector, reflect_dom_object};
 use crate::dom::bindings::root::DomRoot;
 use crate::dom::globalscope::GlobalScope;
@@ -20,6 +22,7 @@ use crate::dom::promise::Promise;
 use crate::realms::{AlreadyInRealm, InRealm};
 use crate::routed_promise::{RoutedPromiseListener, route_promise};
 use crate::script_runtime::CanGc;
+use crate::script_thread::ScriptThread;
 
 #[dom_struct]
 pub(crate) struct ServoInternals {
@@ -55,6 +58,51 @@ impl ServoInternalsMethods<crate::DomTypeHolder> for ServoInternals {
         }
         promise
     }
+
+    /// <https://servo.org/internal-no-spec>
+    fn GetBoolPreference(&self, name: USVString) -> Fallible<bool> {
+        if let PrefValue::Bool(b) = prefs::get().get_value(&name) {
+            return Ok(b);
+        }
+        Err(Error::TypeMismatch)
+    }
+
+    /// <https://servo.org/internal-no-spec>
+    fn GetIntPreference(&self, name: USVString) -> Fallible<i64> {
+        if let PrefValue::Int(i) = prefs::get().get_value(&name) {
+            return Ok(i);
+        }
+        Err(Error::TypeMismatch)
+    }
+
+    /// <https://servo.org/internal-no-spec>
+    fn GetStringPreference(&self, name: USVString) -> Fallible<USVString> {
+        if let PrefValue::Str(s) = prefs::get().get_value(&name) {
+            return Ok(s.into());
+        }
+        Err(Error::TypeMismatch)
+    }
+
+    /// <https://servo.org/internal-no-spec>
+    fn SetBoolPreference(&self, name: USVString, value: bool) {
+        let mut current_prefs = prefs::get().clone();
+        current_prefs.set_value(&name, value.into());
+        prefs::set(current_prefs);
+    }
+
+    /// <https://servo.org/internal-no-spec>
+    fn SetIntPreference(&self, name: USVString, value: i64) {
+        let mut current_prefs = prefs::get().clone();
+        current_prefs.set_value(&name, value.into());
+        prefs::set(current_prefs);
+    }
+
+    /// <https://servo.org/internal-no-spec>
+    fn SetStringPreference(&self, name: USVString, value: USVString) {
+        let mut current_prefs = prefs::get().clone();
+        current_prefs.set_value(&name, value.0.into());
+        prefs::set(current_prefs);
+    }
 }
 
 impl RoutedPromiseListener<MemoryReportResult> for ServoInternals {
@@ -66,14 +114,16 @@ impl RoutedPromiseListener<MemoryReportResult> for ServoInternals {
 }
 
 impl ServoInternalsHelpers for ServoInternals {
-    /// The navigator.servo api is only exposed to about: pages except about:blank
+    /// The navigator.servo api is exposed to about: pages except about:blank, as
+    /// well as any URLs provided by embedders that register new protocol handlers.
     #[allow(unsafe_code)]
     fn is_servo_internal(cx: JSContext, _global: HandleObject) -> bool {
         unsafe {
             let in_realm_proof = AlreadyInRealm::assert_for_cx(cx);
             let global_scope = GlobalScope::from_context(*cx, InRealm::Already(&in_realm_proof));
             let url = global_scope.get_url();
-            url.scheme() == "about" && url.as_str() != "about:blank"
+            (url.scheme() == "about" && url.as_str() != "about:blank") ||
+                ScriptThread::is_servo_privileged(url)
         }
     }
 }

components/script/script_thread.rs

@@ -346,6 +346,10 @@ pub struct ScriptThread {
     needs_rendering_update: Arc<AtomicBool>,
 
     debugger_global: Dom<DebuggerGlobalScope>,
+
+    /// A list of URLs that can access privileged internal APIs.
+    #[no_trace]
+    privileged_urls: Vec<ServoUrl>,
 }
 
 struct BHMExitSignal {
@@ -1016,6 +1020,7 @@ impl ScriptThread {
             scheduled_update_the_rendering: Default::default(),
             needs_rendering_update: Arc::new(AtomicBool::new(false)),
             debugger_global: debugger_global.as_traced(),
+            privileged_urls: state.privileged_urls,
         }
     }
 
@@ -4030,6 +4035,10 @@ impl ScriptThread {
         };
         document.event_handler().handle_refresh_cursor();
     }
+
+    pub(crate) fn is_servo_privileged(url: ServoUrl) -> bool {
+        with_script_thread(|script_thread| script_thread.privileged_urls.contains(&url))
+    }
 }
 
 impl Drop for ScriptThread {