fix: install resize border when set decorations

src/browser_window.rs

@@ -779,6 +779,8 @@ impl BrowserWindow {
   #[napi]
   pub fn set_resizable(&self, resizable: bool) {
     self.window.set_resizable(resizable);
+    // Note: resize border installation is managed by decoration state only.
+    // The resizable flag controls whether the window actually responds to resize attempts.
   }
 
   #[napi]
@@ -1632,6 +1634,14 @@ impl BrowserWindow {
   #[napi]
   pub fn set_decorations(&self, enabled: bool) {
     self.window.set_decorations(enabled);
+
+    // On Windows, when switching to undecorated mode with resizable window,
+    // install the resize border subclass for proper edge resizing support.
+    // This must be done AFTER set_decorations because it may remove WS_THICKFRAME.
+    #[cfg(target_os = "windows")]
+    if !enabled && self.window.is_resizable() {
+      crate::win32_resize::install_resize_border(&self.window);
+    }
   }
 
   #[napi(getter)]

src/win32_resize.rs

@@ -25,6 +25,12 @@ extern "system" {
     dw_ref_data: usize,
   ) -> BOOL;
 
+  fn RemoveWindowSubclass(
+    hwnd: HWND,
+    pfn_subclass: SubclassProc,
+    uid_subclass: usize,
+  ) -> BOOL;
+
   fn DefSubclassProc(hwnd: HWND, umsg: u32, wparam: WPARAM, lparam: LPARAM) -> LRESULT;
 }
 
@@ -36,11 +42,20 @@ unsafe extern "system" fn subclass_proc(
   _uid: usize,
   _dw_ref_data: usize,
 ) -> LRESULT {
-  // WM_NCCALCSIZE: collapse the non-client area so the client rect fills the
-  // entire window.  This keeps the window visually borderless while
-  // WS_THICKFRAME remains in the style (required for drag_resize_window).
+  // WM_NCCALCSIZE: collapse the non-client area but keep 1px border for system shadow.
+  // This keeps WS_THICKFRAME in the style (required for drag_resize_window) while
+  // maintaining the drop shadow around the window.
   if msg == WM_NCCALCSIZE && wparam != 0 {
-    return 0;
+    use windows_sys::Win32::UI::WindowsAndMessaging::NCCALCSIZE_PARAMS;
+    let params = &mut *(lparam as *mut NCCALCSIZE_PARAMS);
+
+    // Keep 1px on all sides for shadow, but remove the rest of the non-client area
+    params.rgrc[0].left += 1;
+    params.rgrc[0].top += 1;
+    params.rgrc[0].right -= 1;
+    params.rgrc[0].bottom -= 1;
+
+    return 0; // Use our calculated client area
   }
 
   DefSubclassProc(hwnd, msg, wparam, lparam)
@@ -61,14 +76,53 @@ pub fn install_resize_border(window: &winit::window::Window) {
 
   unsafe {
     let style = GetWindowLongPtrW(hwnd, GWL_STYLE);
-    // LONG_PTR is i32 on 32-bit and isize on 64-bit; use usize as a
-    // common intermediary for the bit-OR, then let `as _` coerce back.
+
+    // Only add WS_THICKFRAME if not already present
+    if (style as usize & WS_THICKFRAME as usize) == 0 {
+      SetWindowLongPtrW(
+        hwnd,
+        GWL_STYLE,
+        (style as usize | WS_THICKFRAME as usize) as _,
+      );
+      // Force a frame recalculation so WS_THICKFRAME takes effect immediately.
+      SetWindowPos(
+        hwnd,
+        0,
+        0,
+        0,
+        0,
+        0,
+        SWP_NOMOVE | SWP_NOSIZE | SWP_NOZORDER | SWP_FRAMECHANGED,
+      );
+    }
+
+    // Always ensure the subclass is installed (SetWindowSubclass is idempotent with same uid)
+    SetWindowSubclass(hwnd, subclass_proc, 1, 0);
+  }
+}
+
+/// Remove the resize border subclass and WS_THICKFRAME when window becomes non-resizable.
+pub fn uninstall_resize_border(window: &winit::window::Window) {
+  let Ok(handle) = window.window_handle() else {
+    return;
+  };
+  let RawWindowHandle::Win32(h) = handle.as_raw() else {
+    return;
+  };
+  let hwnd = h.hwnd.get() as HWND;
+
+  unsafe {
+    // Remove the subclass
+    RemoveWindowSubclass(hwnd, subclass_proc, 1);
+
+    // Remove WS_THICKFRAME
+    let style = GetWindowLongPtrW(hwnd, GWL_STYLE);
     SetWindowLongPtrW(
       hwnd,
       GWL_STYLE,
-      (style as usize | WS_THICKFRAME as usize) as _,
+      (style as usize & !(WS_THICKFRAME as usize)) as _,
     );
-    // Force a frame recalculation so WS_THICKFRAME takes effect immediately.
+    // Force a frame recalculation
     SetWindowPos(
       hwnd,
       0,
@@ -78,6 +132,5 @@ pub fn install_resize_border(window: &winit::window::Window) {
       0,
       SWP_NOMOVE | SWP_NOSIZE | SWP_NOZORDER | SWP_FRAMECHANGED,
     );
-    SetWindowSubclass(hwnd, subclass_proc, 1, 0);
   }
 }