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Win32: Better DPI changes handling #235

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prokopyl May 1, 2026
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prokopyl May 2, 2026
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prokopyl May 2, 2026
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141 changes: 75 additions & 66 deletions src/win/window.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,14 +9,14 @@ use winapi::um::winuser::{
GetDpiForWindow, GetFocus, GetMessageW, GetWindowLongPtrW, LoadCursorW, PostMessageW,
RegisterClassW, ReleaseCapture, SetCapture, SetCursor, SetFocus, SetProcessDpiAwarenessContext,
SetTimer, SetWindowLongPtrW, SetWindowPos, TrackMouseEvent, TranslateMessage, UnregisterClassW,
CS_OWNDC, GET_XBUTTON_WPARAM, GWLP_USERDATA, HTCLIENT, IDC_ARROW, MSG, SWP_NOMOVE,
SWP_NOZORDER, TRACKMOUSEEVENT, WHEEL_DELTA, WM_CHAR, WM_CLOSE, WM_CREATE, WM_DPICHANGED,
WM_INPUTLANGCHANGE, WM_KEYDOWN, WM_KEYUP, WM_LBUTTONDOWN, WM_LBUTTONUP, WM_MBUTTONDOWN,
WM_MBUTTONUP, WM_MOUSEHWHEEL, WM_MOUSELEAVE, WM_MOUSEMOVE, WM_MOUSEWHEEL, WM_NCDESTROY,
WM_RBUTTONDOWN, WM_RBUTTONUP, WM_SETCURSOR, WM_SHOWWINDOW, WM_SIZE, WM_SYSCHAR, WM_SYSKEYDOWN,
WM_SYSKEYUP, WM_TIMER, WM_USER, WM_XBUTTONDOWN, WM_XBUTTONUP, WNDCLASSW, WS_CAPTION, WS_CHILD,
WS_CLIPSIBLINGS, WS_MAXIMIZEBOX, WS_MINIMIZEBOX, WS_POPUPWINDOW, WS_SIZEBOX, WS_VISIBLE,
XBUTTON1, XBUTTON2,
CS_OWNDC, GET_XBUTTON_WPARAM, GWLP_USERDATA, HTCLIENT, IDC_ARROW, MSG, SWP_NOACTIVATE,
SWP_NOMOVE, SWP_NOZORDER, TRACKMOUSEEVENT, WHEEL_DELTA, WM_CHAR, WM_CLOSE, WM_CREATE,
WM_DPICHANGED, WM_INPUTLANGCHANGE, WM_KEYDOWN, WM_KEYUP, WM_LBUTTONDOWN, WM_LBUTTONUP,
WM_MBUTTONDOWN, WM_MBUTTONUP, WM_MOUSEHWHEEL, WM_MOUSELEAVE, WM_MOUSEMOVE, WM_MOUSEWHEEL,
WM_NCDESTROY, WM_RBUTTONDOWN, WM_RBUTTONUP, WM_SETCURSOR, WM_SHOWWINDOW, WM_SIZE, WM_SYSCHAR,
WM_SYSKEYDOWN, WM_SYSKEYUP, WM_TIMER, WM_USER, WM_XBUTTONDOWN, WM_XBUTTONUP, WNDCLASSW,
WS_CAPTION, WS_CHILD, WS_CLIPSIBLINGS, WS_MAXIMIZEBOX, WS_MINIMIZEBOX, WS_POPUPWINDOW,
WS_SIZEBOX, WS_VISIBLE, XBUTTON1, XBUTTON2,
};

use std::cell::{Cell, Ref, RefCell, RefMut};
Expand Down Expand Up @@ -201,7 +201,7 @@ unsafe fn wnd_proc_inner(
let y = ((lparam >> 16) & 0xFFFF) as i16 as i32;

let physical_pos = PhyPoint { x, y };
let logical_pos = physical_pos.to_logical(&window_state.window_info.borrow());
let logical_pos = physical_pos.to_logical(&window_state.window_info());
let move_event = Event::Mouse(MouseEvent::CursorMoved {
position: logical_pos,
modifiers: window_state
Expand Down Expand Up @@ -363,18 +363,17 @@ unsafe fn wnd_proc_inner(
let height = ((lparam >> 16) & 0xFFFF) as u16 as u32;

let new_window_info = {
let mut window_info = window_state.window_info.borrow_mut();
let new_window_info =
WindowInfo::from_physical_size(PhySize { width, height }, window_info.scale());
let new_size = PhySize { width, height };
let current_size = window_state.current_size.get();

// Only send the event if anything changed
if window_info.physical_size() == new_window_info.physical_size() {
if current_size == new_size {
return None;
}

*window_info = new_window_info;
window_state.current_size.set(new_size);

new_window_info
WindowInfo::from_physical_size(new_size, window_state.current_scale_factor.get())
};

window_state
Expand All @@ -387,46 +386,52 @@ unsafe fn wnd_proc_inner(
None
}
WM_DPICHANGED => {
// To avoid weirdness with the realtime borrow checker.
let new_rect = {
if let WindowScalePolicy::SystemScaleFactor = window_state.scale_policy {
let dpi = (wparam & 0xFFFF) as u16 as u32;
let scale_factor = dpi as f64 / 96.0;

let mut window_info = window_state.window_info.borrow_mut();
*window_info =
WindowInfo::from_logical_size(window_info.logical_size(), scale_factor);

Some((
RECT {
left: 0,
top: 0,
// todo: check if usize fits into i32
right: window_info.physical_size().width as i32,
bottom: window_info.physical_size().height as i32,
},
window_state.dw_style,
))
} else {
None
}
let new_rect = (lparam as *const RECT).read();

let current_size = window_state.current_size.get();
let new_size = PhySize {
width: (new_rect.right - new_rect.left) as u32,
height: (new_rect.bottom - new_rect.top) as u32,
};
if let Some((mut new_rect, dw_style)) = new_rect {
// Convert this desired "client rectangle" size to the actual "window rectangle"
// size (Because of course you have to do that).
AdjustWindowRectEx(&mut new_rect, dw_style, 0, 0);

// Windows makes us resize the window manually. This will trigger another `WM_SIZE` event,
// which we can then send the user the new scale factor.
SetWindowPos(
hwnd,
hwnd,
new_rect.left,
new_rect.top,
new_rect.right - new_rect.left,
new_rect.bottom - new_rect.top,
SWP_NOZORDER | SWP_NOMOVE,
let mut changed = current_size != new_size;

if let WindowScalePolicy::SystemScaleFactor = window_state.scale_policy {
let dpi = (wparam & 0xFFFF) as u16 as u32;
let scale_factor = dpi as f64 / 96.0;

changed |= window_state.current_scale_factor.get() != scale_factor;

window_state.current_scale_factor.set(scale_factor);
}

// Windows makes us resize the window manually. This however will not send a WM_SIZE event,
// hence why we are notifying the window handler manually below.
SetWindowPos(
hwnd,
null_mut(),
new_rect.left,
new_rect.top,
new_rect.right - new_rect.left,
new_rect.bottom - new_rect.top,
SWP_NOZORDER | SWP_NOACTIVATE,
);

if changed {
window_state.current_size.set(new_size);

let mut window = crate::Window::new(window_state.create_window());
let new_window_info = WindowInfo::from_physical_size(
new_size,
window_state.current_scale_factor.get(),
);

window_state
.handler
.borrow_mut()
.as_mut()
.unwrap()
.on_event(&mut window, Event::Window(WindowEvent::Resized(new_window_info)));
}

None
Expand Down Expand Up @@ -496,7 +501,8 @@ pub(super) struct WindowState {
/// GWLP_USERDATA) } as *const WindowState`.
pub hwnd: HWND,
window_class: ATOM,
window_info: RefCell<WindowInfo>,
current_size: Cell<PhySize>,
current_scale_factor: Cell<f64>,
_parent_handle: Option<ParentHandle>,
keyboard_state: RefCell<KeyboardState>,
mouse_button_counter: Cell<usize>,
Expand Down Expand Up @@ -529,8 +535,8 @@ impl WindowState {
Window { state: self }
}

pub(super) fn window_info(&self) -> Ref<'_, WindowInfo> {
self.window_info.borrow()
pub(super) fn window_info(&self) -> WindowInfo {
WindowInfo::from_physical_size(self.current_size.get(), self.current_scale_factor.get())
}

pub(super) fn keyboard_state(&self) -> Ref<'_, KeyboardState> {
Expand All @@ -547,7 +553,7 @@ impl WindowState {
WindowTask::Resize(size) => {
// `self.window_info` will be modified in response to the `WM_SIZE` event that
// follows the `SetWindowPos()` call
let scaling = self.window_info.borrow().scale();
let scaling = self.current_scale_factor.get();
let window_info = WindowInfo::from_logical_size(size, scaling);

// If the window is a standalone window then the size needs to include the window
Expand Down Expand Up @@ -650,14 +656,14 @@ impl Window<'_> {
WindowScalePolicy::ScaleFactor(scale) => scale,
};

let window_info = WindowInfo::from_logical_size(options.size, scaling);
let current_size = WindowInfo::from_logical_size(options.size, scaling).physical_size();

let mut rect = RECT {
left: 0,
top: 0,
// todo: check if usize fits into i32
right: window_info.physical_size().width as i32,
bottom: window_info.physical_size().height as i32,
right: current_size.width as i32,
bottom: current_size.height as i32,
};

let flags = if parented {
Expand Down Expand Up @@ -709,7 +715,8 @@ impl Window<'_> {
let window_state = Rc::new(WindowState {
hwnd,
window_class,
window_info: RefCell::new(window_info),
current_scale_factor: scaling.into(),
current_size: current_size.into(),
_parent_handle: parent_handle,
keyboard_state: RefCell::new(KeyboardState::new()),
mouse_button_counter: Cell::new(0),
Expand Down Expand Up @@ -748,17 +755,19 @@ impl Window<'_> {
let dpi = GetDpiForWindow(hwnd);
let scale_factor = dpi as f64 / 96.0;

let mut window_info = window_state.window_info.borrow_mut();
if window_info.scale() != scale_factor {
*window_info =
WindowInfo::from_logical_size(window_info.logical_size(), scale_factor);
let current_scale_factor = window_state.current_scale_factor.get();

if current_scale_factor != scale_factor {
window_state.current_scale_factor.set(scale_factor);

let current_size = window_state.current_size.get();

Some(RECT {
left: 0,
top: 0,
// todo: check if usize fits into i32
right: window_info.physical_size().width as i32,
bottom: window_info.physical_size().height as i32,
right: current_size.width as i32,
bottom: current_size.height as i32,
})
} else {
None
Expand Down
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