LSP: Initialize platform before reader thread

Closes slint-ui#10274

Author: LeonMatthes

tools/lsp/preview/connector/native.rs

@@ -196,19 +196,29 @@ impl Default for RemoteControlledPreviewToLsp {
 }
 
 impl RemoteControlledPreviewToLsp {
-    /// Creates a RemoteConfrolledPreviewToLsp connector.
+    /// Creates a RemoteControlledPreviewToLsp connector.
     ///
     /// This means the applications lifetime is bound to the lifetime of the
     /// application's STDIN: We quit as soon as that gets fishy or closed.
     ///
     /// It also means we do not need to join the reader thread: The OS will clean
     /// that one up for us anyway.
+    ///
+    /// Note: If the Slint backend has not been set yet, this will set a backend with the
+    /// default Slint BackendSelector.
     pub fn new() -> Self {
         let _ = Self::process_input();
         Self {}
     }
 
     fn process_input() -> std::thread::JoinHandle<std::result::Result<(), String>> {
+        // Ensure the backend is set up before the reader thread starts. This fixes
+        // bug #10274 on macOS where a race condition was causing the reader thread to already
+        // process messages before the event loop was running.
+        //
+        // Use .ok() to ignore any errors, as the backend might already be set by the user and that's fine.
+        slint::BackendSelector::new().select().ok();
+
         std::thread::spawn(move || -> Result<(), String> {
             let reader = std::io::BufReader::new(std::io::stdin().lock());
             for line in reader.lines() {