Auto merge of #18777 - mateon1:perf/headless-event-poll, r=jdm

Improved headless Servo performance

Now the main thread doesn't waste 100% CPU, polling for events.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #18770 (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because: I verified Servo works correctly in headless mode, and the linked issue is gone.

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ports/glutin/window.rs

@@ -39,6 +39,8 @@ use std::mem;
 use std::os::raw::c_void;
 use std::ptr;
 use std::rc::Rc;
+use std::thread;
+use std::time;
 use style_traits::DevicePixel;
 use style_traits::cursor::Cursor;
 #[cfg(target_os = "windows")]
@@ -684,7 +686,13 @@ impl Window {
                         close_event = self.handle_window_event(event) || close_event;
                     }
                 }
-                WindowKind::Headless(..) => {}
+                WindowKind::Headless(..) => {
+                    // Sleep the main thread to avoid using 100% CPU
+                    // This can be done better, see comments in #18777
+                    if events.is_empty() {
+                        thread::sleep(time::Duration::from_millis(5));
+                    }
+                }
             }
         } else {
             close_event = self.handle_next_event();