Fix #867: macOS file dialogs no longer run inside the window's event handler

On macOS the Open and Save panels ran modally on the UI thread, inside
the window's event handler. Resizing the panel delivered window events
to that handler again, and the app crashed when the mouse was released.
They now open from a thread of their own, as on Linux (#592): rfd runs
the panel on the main thread from the run loop, outside the handler, and
the app picks the answer up when it comes.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/vectorcraft/src/main.rs

@@ -274,9 +274,12 @@ fn write_file(path: &str, bytes: &[u8]) -> Result<(), String> {
     fileio::write_atomic(std::path::Path::new(path), bytes).map_err(|e| e.to_string())
 }
 
-/// Linux: show the dialog for `request` over `parent` on a thread of its own, answering on the
-/// receiver. Shown on the UI thread, nothing would answer the compositor meanwhile, which then
-/// offers to kill the window as not responding (#592).
+/// Linux and macOS: show the dialog for `request` over `parent` on a thread of its own, answering
+/// on the receiver. Shown on the UI thread, nothing would answer the compositor meanwhile on Linux,
+/// which then offers to kill the window as not responding (#592); on macOS the panel ran modally
+/// inside the window's event handler, and resizing it delivered window events to that handler
+/// again, which crashed the app (#867). From another thread, rfd runs the panel on the main thread
+/// from the run loop, outside the handler.
 fn start_pick(request: PickRequest, parent: &Parent) -> Option<std::sync::mpsc::Receiver<Vec<String>>> {
     let dialog = file_dialog(&request, parent);
     let (tx, rx) = std::sync::mpsc::channel();
@@ -296,9 +299,9 @@ fn services(parent: Parent) -> Services {
         pick_open: Some(Box::new(move |pick: &FilePick| pick_now(PickRequest::Open(pick.clone()), &p1).into_iter().next())),
         pick_open_multi: Some(Box::new(move || pick_now(PickRequest::OpenMany, &p2))),
         pick_save: Some(Box::new(move |pick: &FilePick| pick_now(PickRequest::Save(pick.clone()), &p3).into_iter().next())),
-        // Windows and macOS dialogs run the window's events while they are open; Linux's don't.
-        start_pick: cfg!(all(unix, not(target_os = "macos")))
-            .then(|| Box::new(move |request: PickRequest| start_pick(request, &p5)) as vectorcraft_ui_egui::picks::StartPick),
+        // Windows dialogs run the window's events while they are open; Linux's don't, and macOS
+        // ones re-enter the window's event handler (#867).
+        start_pick: cfg!(unix).then(|| Box::new(move |request: PickRequest| start_pick(request, &p5)) as vectorcraft_ui_egui::picks::StartPick),
         read: Some(Box::new(|p: &str| std::fs::read(p).map_err(|e| e.to_string()))),
         write: Some(Box::new(write_file)),
         // Background Save and Export write from a worker thread.