feat(windows): Enable native resizing on undecorated windows with shadows

src/platform_impl/windows/event_loop.rs

@@ -23,7 +23,8 @@ use windows::{
   core::{s, PCWSTR},
   Win32::{
     Foundation::{
-      BOOL, HANDLE, HINSTANCE, HMODULE, HWND, LPARAM, LRESULT, POINT, RECT, WAIT_TIMEOUT, WPARAM,
+      BOOL, FALSE, HANDLE, HINSTANCE, HMODULE, HWND, LPARAM, LRESULT, POINT, RECT, WAIT_TIMEOUT,
+      WPARAM,
     },
     Graphics::Gdi::*,
     System::{
@@ -2121,9 +2122,27 @@ unsafe fn public_window_callback_inner<T: 'static>(
             params.rgrc[0] = rect;
           }
         } else if window_flags.contains(WindowFlags::MARKER_UNDECORATED_SHADOW) {
+          let mut rect = RECT::default();
+          if AdjustWindowRectEx(
+            &mut rect,
+            WINDOW_STYLE((GetWindowLongPtrW(window, GWL_STYLE) & !(WS_CAPTION.0 as isize)) as u32),
+            FALSE,
+            WINDOW_EX_STYLE::default(),
+          )
+          .is_ok()
+          {
+            rect.left *= -1;
+            rect.top *= -1;
+          } else {
+            // As a fallback value, this keeps the shadow border but disables the resizing.
+            rect.bottom = -1;
+          }
+
           let params = &mut *(lparam.0 as *mut NCCALCSIZE_PARAMS);
           params.rgrc[0].top += 1;
-          params.rgrc[0].bottom += 1;
+          params.rgrc[0].left += rect.left;
+          params.rgrc[0].right -= rect.right;
+          params.rgrc[0].bottom -= rect.bottom;
         }
         result = ProcResult::Value(LRESULT(0)); // return 0 here to make the window borderless
       }

src/platform_impl/windows/window.rs

@@ -182,6 +182,24 @@ impl Window {
     }
   }
 
+  #[inline]
+  pub(crate) fn request_nccalcsize(&self) {
+    unsafe {
+      let mut rect = RECT::default();
+      if GetWindowRect(self.window.0, &mut rect).is_ok() {
+        let _ = SetWindowPos(
+          self.window.0,
+          HWND::default(),
+          rect.left,
+          rect.top,
+          rect.right - rect.left,
+          rect.bottom - rect.top,
+          SWP_FRAMECHANGED,
+        );
+      }
+    }
+  }
+
   #[inline]
   pub fn outer_position(&self) -> Result<PhysicalPosition<i32>, NotSupportedError> {
     unsafe { util::get_window_rect(self.window.0) }

src/window.rs

@@ -556,6 +556,8 @@ impl WindowBuilder {
   ) -> Result<Window, OsError> {
     platform_impl::Window::new(&window_target.p, self.window, self.platform_specific).map(
       |window| {
+        #[cfg(windows)]
+        window.request_nccalcsize();
         window.request_redraw();
         Window { window }
       },