Bug 1880394 - Give the icon to the dialog window class, and revert bug 1880226. r=win-reviewers,handyman

This is to be landed after the merge. I think this is better / more
consistent, and avoids relying on mis-documented window style flags in
MSDN.

Differential Revision: https://phabricator.services.mozilla.com/D201894

widget/windows/nsWindow.cpp

@@ -1221,18 +1221,19 @@ static LPWSTR const gStockApplicationIcon = MAKEINTRESOURCEW(32512);
 
 /* static */
 const wchar_t* nsWindow::ChooseWindowClass(WindowType aWindowType) {
-  switch (aWindowType) {
-    case WindowType::Invisible:
-      return RegisterWindowClass(kClassNameHidden, 0, gStockApplicationIcon);
-    case WindowType::Dialog:
-      return RegisterWindowClass(kClassNameDialog, 0, nullptr);
-    case WindowType::Popup:
-      return RegisterWindowClass(kClassNameDropShadow, 0,
-                                 gStockApplicationIcon);
-    default:
-      return RegisterWindowClass(GetMainWindowClass(), 0,
-                                 gStockApplicationIcon);
-  }
+  const wchar_t* className = [aWindowType] {
+    switch (aWindowType) {
+      case WindowType::Invisible:
+        return kClassNameHidden;
+      case WindowType::Dialog:
+        return kClassNameDialog;
+      case WindowType::Popup:
+        return kClassNameDropShadow;
+      default:
+        return GetMainWindowClass();
+    }
+  }();
+  return RegisterWindowClass(className, 0, gStockApplicationIcon);
 }
 
 /**************************************************************
@@ -1332,7 +1333,6 @@ DWORD nsWindow::WindowStyle() {
 
 // Return nsWindow extended styles
 DWORD nsWindow::WindowExStyle() {
-  MOZ_ASSERT_IF(mIsAlert, mWindowType == WindowType::Dialog);
   switch (mWindowType) {
     case WindowType::Child:
       return 0;
@@ -1343,18 +1343,18 @@ DWORD nsWindow::WindowExStyle() {
       }
       return extendedStyle;
     }
-    case WindowType::Dialog: {
-      if (mIsAlert) {
-        return WS_EX_TOOLWINDOW;
-      }
-      return WS_EX_WINDOWEDGE | WS_EX_DLGMODALFRAME;
-    }
     case WindowType::Sheet:
       MOZ_FALLTHROUGH_ASSERT("Sheets are macOS specific");
+    case WindowType::Dialog:
     case WindowType::TopLevel:
     case WindowType::Invisible:
       break;
   }
+  if (mIsAlert) {
+    MOZ_ASSERT(mWindowType == WindowType::Dialog,
+               "Expect alert windows to have type=dialog");
+    return WS_EX_TOOLWINDOW;
+  }
   return WS_EX_WINDOWEDGE;
 }