container: predict window size for initial configure

previously, windows were allowed determine their own initial dimensions,
then fed a resize afterwards. we now look up the parent container and
try to predict + report the real size up front to minimise visual
artifacting on spawn/destroy

src/state.rs

@@ -1597,6 +1597,28 @@ impl State {
         self.config.get()?.initial_tile_state(data)
     }
 
+    pub fn predict_tiled_body_size(&self) -> Option<(i32, i32)> {
+        let seat = self.seat_queue.last();
+        let ws = self.ensure_map_workspace(seat.as_deref());
+        let pos = ws.position.get();
+        if pos.is_empty() {
+            return None;
+        }
+        if let Some(c) = ws.container.get() {
+            let la = c.clone().tl_last_active_child();
+            let target = la
+                .tl_data()
+                .parent
+                .get()
+                .and_then(|n| n.node_into_container())
+                .unwrap_or(c);
+            Some(target.predict_child_body_size())
+        } else {
+            let tpuh = self.theme.title_plus_underline_height();
+            Some((pos.width(), (pos.height() - tpuh).max(0)))
+        }
+    }
+
     pub fn update_capabilities(
         &self,
         data: &Rc<Client>,