fix(cef): fix start_window_dragging on Windows (#14618)

Author: amrbashir

crates/tauri-runtime-cef/src/cef_impl.rs

@@ -30,7 +30,9 @@ use crate::{
 };
 
 mod cookie;
+mod drag_window;
 mod request_handler;
+
 use cookie::{CollectAllCookiesVisitor, CollectUrlCookiesVisitor};
 
 #[cfg(target_os = "linux")]
@@ -823,6 +825,11 @@ wrap_window_delegate! {
   impl WindowDelegate {
     fn on_window_created(&self, window: Option<&mut Window>) {
       if let Some(window) = window {
+
+        // Setup necessary handling for `start_window_dragging` to work on Windows
+        #[cfg(windows)]
+        drag_window::windows::subclass_window_for_dragging(window);
+
         let a = self.attributes.borrow();
         if let Some(icon) = a.icon.clone() {
           set_window_icon(window, icon);
@@ -1884,16 +1891,28 @@ fn start_window_dragging(window: &cef::Window) {
 
 #[cfg(windows)]
 fn start_window_dragging(window: &cef::Window) {
-  use windows::Win32::Foundation::HWND;
-  use windows::Win32::UI::WindowsAndMessaging::{SendMessageW, HTCAPTION, WM_NCLBUTTONDOWN};
+  use windows::Win32::Foundation::*;
+  use windows::Win32::UI::Input::KeyboardAndMouse::*;
+  use windows::Win32::UI::WindowsAndMessaging::*;
 
   unsafe {
     let hwnd = window.window_handle();
-    let _ = SendMessageW(
-      HWND(hwnd.0 as _),
+
+    let mut pos = std::mem::zeroed();
+    let _ = GetCursorPos(&mut pos);
+
+    let points = POINTS {
+      x: pos.x as i16,
+      y: pos.y as i16,
+    };
+
+    let _ = ReleaseCapture();
+
+    let _ = PostMessageW(
+      Some(HWND(hwnd.0 as _)),
       WM_NCLBUTTONDOWN,
-      Some(windows::Win32::Foundation::WPARAM(HTCAPTION as usize)),
-      Some(windows::Win32::Foundation::LPARAM(0)),
+      WPARAM(HTCAPTION as usize),
+      LPARAM(&points as *const _ as isize),
     );
   }
 }

crates/tauri-runtime-cef/src/cef_impl/drag_window.rs

@@ -0,0 +1,71 @@
+// Copyright 2019-2024 Tauri Programme within The Commons Conservancy
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: MIT
+
+pub mod windows {
+  use cef::*;
+  use windows::core::{w, PCWSTR};
+  use windows::Win32::Foundation::*;
+  use windows::Win32::UI::WindowsAndMessaging::*;
+
+  /// Same as [WNDPROC] but without the Option wrapper.
+  type WindowProc = unsafe extern "system" fn(HWND, u32, WPARAM, LPARAM) -> LRESULT;
+
+  const ORIGINAL_WND_PROP: PCWSTR = w!("TAURI_CEF_ORIGINAL_WND_PROC");
+
+  /// Subclasses the given window to handle draggable regions
+  /// by replacing its window procedure with `root_window_proc`
+  /// and storing the original procedure as a property to be called later.
+  pub fn subclass_window_for_dragging(window: &mut cef::Window) {
+    let hwnd = window.window_handle();
+    let hwnd = HWND(hwnd.0 as _);
+    subclass_window(hwnd, root_window_proc);
+  }
+
+  /// Subclasses a window by replacing its window procedure with the given `proc`
+  /// and storing the original procedure as a property for later use.
+  fn subclass_window(hwnd: HWND, proc: WindowProc) {
+    // If already subclassed, return early
+    let orginial_wnd_proc = unsafe { GetPropW(hwnd, ORIGINAL_WND_PROP) };
+    if !orginial_wnd_proc.is_invalid() {
+      return;
+    }
+
+    // Reset last error
+    unsafe { SetLastError(ERROR_SUCCESS) };
+
+    // Set the new window procedure and get the orginal one
+    let original_wnd_proc = unsafe { SetWindowLongPtrW(hwnd, GWLP_WNDPROC, proc as isize) };
+    if original_wnd_proc == 0 && unsafe { GetLastError() } != ERROR_SUCCESS {
+      return;
+    }
+
+    unsafe {
+      // Store the original window proc as a property for later use
+      let _ = SetPropW(
+        hwnd,
+        ORIGINAL_WND_PROP,
+        Some(HANDLE(original_wnd_proc as _)),
+      );
+    }
+  }
+
+  /// The root window procedure to handle WM_NCLBUTTONDOWN
+  /// by calling DefWindowProcW directly to allow dragging
+  /// and forwarding other messages to the original CEF window procedure.
+  unsafe extern "system" fn root_window_proc(
+    hwnd: HWND,
+    msg: u32,
+    wparam: WPARAM,
+    lparam: LPARAM,
+  ) -> LRESULT {
+    if msg == WM_NCLBUTTONDOWN {
+      return unsafe { DefWindowProcW(hwnd, msg, wparam, lparam) };
+    }
+
+    // For other messages, call the original CEF window procedure
+    let original_wnd_proc = GetPropW(hwnd, ORIGINAL_WND_PROP);
+    let original_wnd_proc = std::mem::transmute::<_, WindowProc>(original_wnd_proc.0);
+    CallWindowProcW(Some(original_wnd_proc), hwnd, msg, wparam, lparam)
+  }
+}