gpui: Allow OS caption/buttons for custom Windows titlebar

[momota1029]

Summary

Fixes an issue where GPUI's handling of WM_NCLBUTTONDOWN prevented Windows from processing default titlebar interactions (dragging, caption buttons, and border resize).

Changes

• Allow OS to handle WM_NCLBUTTONDOWN events for HTCAPTION, caption button areas, and resize border areas (HTLEFT, HTRIGHT, HTTOP, HTBOTTOM, HTTOPLEFT, HTTOPRIGHT, HTBOTTOMLEFT, HTBOTTOMRIGHT)
• Use current cursor position in WM_NCHITTEST for accurate hit-testing

Testing

• Manual testing on Windows: titlebar dragging works
• Manual testing on Windows: minimize/maximize/close buttons work
• Manual testing on Windows: window border resize works in all directions

Release Notes:

• N/A

[zed-industries-bot]

	Warnings
⚠️	This PR is missing release notes. Please add a "Release Notes" section that describes the change: Release Notes: - Added/Fixed/Improved ... If your change is not user-facing, you can use "N/A" for the entry: Release Notes: - N/A

Generated by 🚫 dangerJS against 2784926

[reflectronic]

Can you explain the behavior you're seeing that you are trying to fix here? We already pass WM_NCLBUTTONDOWN events to the OS if they are not handled by the application. The caption buttons we use in Zed, for example (https://github.com/zed-industries/zed/blob/main/crates/platform_title_bar/src/platforms/platform_windows.rs), are not interactive elements, so handled is already false.

Is this for your own GPUI application? You should be able to make the caption buttons and title bar dragging work using a similar pattern as in the code I linked.

[momota1029]

Yes, this is for my own GPUI application. Thank you for pointing me to the Zed caption button implementation — I was able to narrow down the issue further.

The problem is reproducible with just .track_focus() on a root element. When a focusable element receives a MouseDownEvent, the focus listener calls window.prevent_default() (to prevent parent focusables from stealing focus). This sets default_prevented = true on the DispatchEventResult.

In handle_nc_mouse_down_msg, the check is:

let handled = !result.propagate || result.default_prevented;

Since default_prevented is true, handled becomes true, and Some(0) is returned — DefWindowProcW is never called, so titlebar dragging and caption buttons stop working.

Zed's own titlebar works because platform_title_bar caption buttons are not focusable interactive elements, so handled stays false.

Here is a minimal reproduction. Save it as crates/gpui/examples/custom_titlebar.rs, add the [[example]] entry to crates/gpui/Cargo.toml, and run with cargo run -p gpui --example custom_titlebar. Removing the .track_focus() line makes everything work again.

use gpui::{
    App, Application, Bounds, Context, FocusHandle, KeyBinding, Window, WindowBounds,
    WindowControlArea, WindowOptions, actions, div, prelude::*, px, rgb, size,
};

actions!(custom_titlebar, [Quit]);

struct Demo {
    focus_handle: FocusHandle,
}

impl Render for Demo {
    fn render(&mut self, window: &mut Window, _cx: &mut Context<Self>) -> impl IntoElement {
        div()
            .id("root")
            .track_focus(&self.focus_handle) // <-- causes the bug
            .flex()
            .flex_col()
            .bg(rgb(0x1e1e1e))
            .text_color(rgb(0xdddddd))
            .size_full()
            .child(
                div()
                    .flex()
                    .flex_row()
                    .h(px(32.))
                    .bg(rgb(0x2d2d2d))
                    .child(
                        div()
                            .flex_1()
                            .h_full()
                            .flex()
                            .items_center()
                            .px_2()
                            .window_control_area(WindowControlArea::Drag)
                            .child("Drag here"),
                    )
                    .child(
                        div()
                            .id("minimize")
                            .flex().items_center().justify_center()
                            .w(px(36.)).h_full()
                            .hover(|s| s.bg(rgb(0x555555)))
                            .window_control_area(WindowControlArea::Min)
                            .child("\u{2500}"),
                    )
                    .child(
                        div()
                            .id("maximize")
                            .flex().items_center().justify_center()
                            .w(px(36.)).h_full()
                            .hover(|s| s.bg(rgb(0x555555)))
                            .window_control_area(WindowControlArea::Max)
                            .child(if window.is_maximized() { "\u{25A3}" } else { "\u{25A1}" }),
                    )
                    .child(
                        div()
                            .id("close")
                            .flex().items_center().justify_center()
                            .w(px(36.)).h_full()
                            .hover(|s| s.bg(rgb(0xe81123)))
                            .window_control_area(WindowControlArea::Close)
                            .child("\u{2715}"),
                    ),
            )
            .child(
                div().flex_1().p_4().child(
                    "Try dragging the titlebar or clicking the caption buttons.",
                ),
            )
    }
}

fn main() {
    Application::new().run(|cx: &mut App| {
        cx.on_action(|_: &Quit, cx| cx.quit());
        cx.bind_keys([KeyBinding::new("cmd-q", Quit, None)]);
        cx.open_window(
            WindowOptions {
                titlebar: None,
                window_bounds: Some(WindowBounds::Windowed(Bounds::centered(
                    None, size(px(500.0), px(300.0)), cx,
                ))),
                ..Default::default()
            },
            |_, cx| cx.new(|cx| Demo { focus_handle: cx.focus_handle() }),
        )
        .unwrap();
        cx.activate(true);
    });
}

[reflectronic]

I think the only change needed here, then, is:

-            let result = func(input);
-            let handled = !result.propagate || result.default_prevented;
+            let handled = !func(input).propagate;

I don't understand why default_prevented is in this condition and removing it doesn't seem to cause issues in Zed. The other handlers (e.g. handle_nc_mouse_up_msg) aren't written this way, either.

[momota1029]

Thanks, agreed.

I updated handle_nc_mouse_down_msg to use only propagate as the handled signal:

let handled = !func(input).propagate;

I re-tested with the track_focus() custom-titlebar repro, and titlebar dragging + caption buttons work correctly with this change.

I also reduced the PR to this minimal behavior change.

[reflectronic]

Thank you

[Serophots]

This change has introduced a regression (not in zed but in some GPUI apps) on Windows when you have a button in the navbar draggable area. Seems to work fine for MacOS.

I've modified the above example to include a button. On Windows, when clicked, the event handler is not called anymore.

use gpui::{
    App, Application, Bounds, Context, FocusHandle, KeyBinding, Window, WindowBounds,
    WindowControlArea, WindowOptions, actions, div, prelude::*, px, rgb, size,
};

actions!(custom_titlebar, [Quit]);

struct Demo {
    focus_handle: FocusHandle,
}

impl Render for Demo {
    fn render(&mut self, window: &mut Window, _cx: &mut Context<Self>) -> impl IntoElement {
        div()
            .id("root")
            .track_focus(&self.focus_handle)
            .flex()
            .flex_col()
            .bg(rgb(0x1e1e1e))
            .text_color(rgb(0xdddddd))
            .size_full()
            .child(
                div()
                    .flex()
                    .flex_row()
                    .h(px(32.))
                    .bg(rgb(0x2d2d2d))
                    .child(
                        div()
                            .flex_1()
                            .h_full()
                            .flex()
                            .items_center()
                            .px_2()
                            .window_control_area(WindowControlArea::Drag)
                            .child("Drag here")
                            .child(
                                div()
                                    .child("Button here")
                                    .id("button")
                                    .cursor_pointer()
                                    .on_click(|_, _, _| {
                                        println!("CLICKED");
                                    }),
                            ),
                    )
                    .child(
                        div()
                            .id("minimize")
                            .flex()
                            .items_center()
                            .justify_center()
                            .w(px(36.))
                            .h_full()
                            .hover(|s| s.bg(rgb(0x555555)))
                            .window_control_area(WindowControlArea::Min)
                            .child("\u{2500}"),
                    )
                    .child(
                        div()
                            .id("maximize")
                            .flex()
                            .items_center()
                            .justify_center()
                            .w(px(36.))
                            .h_full()
                            .hover(|s| s.bg(rgb(0x555555)))
                            .window_control_area(WindowControlArea::Max)
                            .child(if window.is_maximized() {
                                "\u{25A3}"
                            } else {
                                "\u{25A1}"
                            }),
                    )
                    .child(
                        div()
                            .id("close")
                            .flex()
                            .items_center()
                            .justify_center()
                            .w(px(36.))
                            .h_full()
                            .hover(|s| s.bg(rgb(0xe81123)))
                            .window_control_area(WindowControlArea::Close)
                            .child("\u{2715}"),
                    ),
            )
            .child(
                div()
                    .flex_1()
                    .p_4()
                    .child("Try dragging the titlebar or clicking the caption buttons."),
            )
    }
}

fn main() {
    Application::new().run(|cx: &mut App| {
        cx.on_action(|_: &Quit, cx| cx.quit());
        cx.bind_keys([KeyBinding::new("cmd-q", Quit, None)]);
        cx.open_window(
            WindowOptions {
                titlebar: None,
                window_bounds: Some(WindowBounds::Windowed(Bounds::centered(
                    None,
                    size(px(500.0), px(300.0)),
                    cx,
                ))),
                ..Default::default()
            },
            |_, cx| {
                cx.new(|cx| Demo {
                    focus_handle: cx.focus_handle(),
                })
            },
        )
        .unwrap();
        cx.activate(true);
    });
}