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/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");