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/ifs/wl_surface/xdg_surface/xdg_toplevel.rs

@@ -780,7 +780,8 @@ impl XdgSurfaceExt for XdgToplevel {
     fn initial_configure(self: Rc<Self>) {
         let rect = self.xdg.absolute_desired_extents.get();
         if rect.is_empty() {
-            self.send_configure(0, 0);
+            let (w, h) = self.state.predict_tiled_body_size().unwrap_or((0, 0));
+            self.send_configure(w, h);
         } else {
             self.send_configure_checked(rect.width(), rect.height());
         }

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>,

src/tree/container.rs

@@ -377,6 +377,32 @@ impl ContainerNode {
         }
     }
 
+    pub fn predict_child_body_size(&self) -> (i32, i32) {
+        let tpuh = self.state.theme.title_plus_underline_height();
+        if self.mono_child.is_some() {
+            let mb = self.mono_body.get();
+            return (mb.width(), mb.height());
+        }
+        let spacing = self.state.theme.sizes.gap.get().max(
+            self.state.theme.sizes.border_width.get(),
+        );
+        let nc = self.num_children.get() as i32 + 1;
+        match self.split.get() {
+            ContainerSplit::Horizontal => {
+                let content_w = self.width.get().sub((nc - 1) * spacing).max(0);
+                (content_w / nc, self.height.get().sub(tpuh).max(0))
+            }
+            ContainerSplit::Vertical => {
+                let content_h = self
+                    .height
+                    .get()
+                    .sub(tpuh + (nc - 1) * (spacing + tpuh))
+                    .max(0);
+                (self.width.get(), content_h / nc)
+            }
+        }
+    }
+
     pub fn on_spaces_changed(self: &Rc<Self>) {
         self.update_content_size();
         // log::info!("on_spaces_changed");