gpui: Support disabling window resizing and minimizing (#36859)

Add support to disable both window resizing and minimizing.

| | macOS | Windows |
| - | - | - |
| **Unresizable** | <img width="412" height="440"
alt="SCR-20250822-qpea"
src="https://github.com/user-attachments/assets/d6d45510-dc4b-436f-a9fa-ce9cb0b0c411"
/> | <img width="276" height="298" alt="2025-08-22 110757"
src="https://github.com/user-attachments/assets/9deff498-e903-4173-9c26-072dd9409fc1"
/> |
| **Unminimizable** | <img width="412" height="440"
alt="SCR-20250822-qpfl"
src="https://github.com/user-attachments/assets/e1d5f9eb-6de5-4908-8b52-38ccb2e65689"
/> | <img width="276" height="298" alt="2025-08-22 110814"
src="https://github.com/user-attachments/assets/da94b006-3544-4274-8b02-1cab7ca8dd70"
/> |

Release Notes:

- N/A

crates/gpui/examples/window.rs

@@ -152,6 +152,36 @@ impl Render for WindowDemo {
                 )
                 .unwrap();
             }))
+            .child(button("Unresizable", move |_, cx| {
+                cx.open_window(
+                    WindowOptions {
+                        is_resizable: false,
+                        window_bounds: Some(window_bounds),
+                        ..Default::default()
+                    },
+                    |_, cx| {
+                        cx.new(|_| SubWindow {
+                            custom_titlebar: false,
+                        })
+                    },
+                )
+                .unwrap();
+            }))
+            .child(button("Unminimizable", move |_, cx| {
+                cx.open_window(
+                    WindowOptions {
+                        is_minimizable: false,
+                        window_bounds: Some(window_bounds),
+                        ..Default::default()
+                    },
+                    |_, cx| {
+                        cx.new(|_| SubWindow {
+                            custom_titlebar: false,
+                        })
+                    },
+                )
+                .unwrap();
+            }))
             .child(button("Hide Application", |window, cx| {
                 cx.hide();
 

crates/gpui/examples/window_positioning.rs

@@ -62,6 +62,7 @@ fn build_window_options(display_id: DisplayId, bounds: Bounds<Pixels>) -> Window
         app_id: None,
         window_min_size: None,
         window_decorations: None,
+        ..Default::default()
     }
 }