Desktop: keep the off-screen check until the monitor size is known (#23) (#76)

apps/wordcraft/src/main.rs

@@ -14,7 +14,7 @@ mod graphics;
 mod logging;
 
 use wordcraft_engine::Session;
-use wordcraft_ui_egui::{Services, UiState, WordApp, window_geometry::WindowGeometry};
+use wordcraft_ui_egui::{Services, UiState, WordApp, window_geometry::{WindowGeometry, take_rescue}};
 
 /// The app, and the restored window geometry until the first frame has checked it.
 struct App(WordApp, Option<WindowGeometry>);
@@ -24,7 +24,7 @@ impl eframe::App for App {
         self.0.logic(ctx);
         let prev = self.0.ui.window;
         self.0.ui.window = ctx.input(|i| WindowGeometry::track(prev, i.viewport(), i.viewport_rect().size()));
-        if let Some(pos) = self.1.take().and_then(|g| ctx.input(|i| g.rescue_position(i.viewport()))) {
+        if let Some(pos) = ctx.input(|i| take_rescue(&mut self.1, i.viewport())) {
             ctx.send_viewport_cmd(egui::ViewportCommand::OuterPosition(pos));
         }
         if self.0.quit_requested {

crates/ui-egui/src/window_geometry.rs

@@ -91,6 +91,14 @@ impl WindowGeometry {
     }
 }
 
+/// Checks the restored geometry in `pending` once, and only once the monitor size is known: while
+/// `info` doesn't report a usable one, `pending` is left in place for a later frame. Then it is taken
+/// and [`WindowGeometry::rescue_position`] decides where to move the window, if anywhere.
+pub fn take_rescue(pending: &mut Option<WindowGeometry>, info: &egui::ViewportInfo) -> Option<egui::Pos2> {
+    info.monitor_size.filter(|s| s.is_finite() && s.x > 0.0 && s.y > 0.0)?;
+    pending.take()?.rescue_position(info)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -174,6 +182,23 @@ mod tests {
         assert_eq!(saved().rescue_position(&egui::ViewportInfo::default()), None);
     }
 
+    #[test]
+    fn the_rescue_waits_for_the_monitor_size() {
+        let on = egui::Rect::from_min_size(egui::pos2(2000.0, 50.0), egui::vec2(1200.0, 800.0));
+        // The first frame doesn't know the monitor yet: nothing happens and the geometry is kept.
+        let mut pending = Some(saved());
+        assert_eq!(take_rescue(&mut pending, &egui::ViewportInfo::default()), None);
+        assert_eq!(pending, Some(saved()));
+        // A later frame reports a different monitor: centred on it, and the check is done.
+        assert_eq!(take_rescue(&mut pending, &info(on, egui::vec2(1600.0, 900.0))), Some(egui::pos2(200.0, 50.0)));
+        assert_eq!(pending, None);
+        assert_eq!(take_rescue(&mut pending, &info(on, egui::vec2(1024.0, 768.0))), None);
+        // The same monitor: no move, and the check is done as well.
+        let mut pending = Some(saved());
+        assert_eq!(take_rescue(&mut pending, &info(on, egui::vec2(1920.0, 1080.0))), None);
+        assert_eq!(pending, None);
+    }
+
     #[test]
     fn survives_the_prefs_round_trip() {
         let ui = crate::UiState { window: Some(saved()), ..Default::default() };