Fix problem with wrongly deleting the background brush

We shouldn't install HBRUSH owned by us as class background brush
because this brush is deleted by Windows itself, invalidating the HBRUSH
and resulting in visible problems when the background is painted using
this HBRUSH after showing a task dialog in dark mode.

Fix this by uninstalling the brush before deleting the window.

This also avoids the awkward code deleting the old class brush only if
it's not one of the standard brushes: now we don't need to do it any
longer, as we restore this brush before deleting the window.

src/msw/taskdlg.cpp

@@ -755,7 +755,7 @@ TDPageSubclassProc(HWND hwnd,
                    WPARAM wParam,
                    LPARAM lParam,
                    UINT_PTR uId,
-                   DWORD_PTR WXUNUSED(dwRef))
+                   DWORD_PTR dwRef)
 {
     switch ( msg )
     {
@@ -834,6 +834,8 @@ TDPageSubclassProc(HWND hwnd,
             break;
 
         case WM_DESTROY:
+            // Restore the original class brush we had changed.
+            ::SetClassLongPtr(hwnd, GCLP_HBRBACKGROUND, dwRef);
             TDPageState::Destroy(hwnd);
             ::RemoveWindowSubclass(hwnd, TDPageSubclassProc, uId);
             break;
@@ -978,7 +980,7 @@ SetWindowSubclassIfNeeded(HWND hwnd,
     if ( ::GetWindowSubclass(hwnd, proc, uId, &dwRef) )
         return;
 
-    dwRef = reinterpret_cast<DWORD_PTR>(initFunc());
+    dwRef = static_cast<DWORD_PTR>(initFunc());
     if ( !::SetWindowSubclass(hwnd, proc, uId, dwRef) )
     {
         wxLogLastError("SetWindowSubclass");
@@ -1022,7 +1024,7 @@ void TDSubclassContainer(HWND hwndParent, COLORREF bg)
         hwndParent,
         TDCtrlContainerSubclassProc,
         kTDCtrlSubclassId,
-        [bg]() { return GetSolidBrush(bg); }
+        [bg]() { return reinterpret_cast<DWORD_PTR>(GetSolidBrush(bg)); }
     );
 }
 
@@ -1154,21 +1156,6 @@ BOOL CALLBACK TDEnumAttachProc(HWND hwndChild, LPARAM lparam)
     if ( !hDUI )
         return TRUE;
 
-    // Class background brush
-    {
-        HBRUSH nb = GetSolidBrush(TDDarkCol::kSecondary);
-
-        HBRUSH ob = reinterpret_cast<HBRUSH>(
-            ::SetClassLongPtr(hDUI, GCLP_HBRBACKGROUND, reinterpret_cast<LONG_PTR>(nb))
-        );
-        if ( ob &&
-                ob != ::GetSysColorBrush(COLOR_WINDOW) &&
-                    ob != ::GetSysColorBrush(COLOR_BTNFACE))
-        {
-            ::DeleteObject(ob);
-        }
-    }
-
     TDApplyToChildren(pEl);
     wxMSWDarkMode::SetTheme(hDUI, L"DarkMode_Explorer");
 
@@ -1183,7 +1170,20 @@ BOOL CALLBACK TDEnumAttachProc(HWND hwndChild, LPARAM lparam)
     s.elemsOk = false;
     TDUpdateLayoutCache(hDUI, s);
 
-    SetWindowSubclassIfNeeded(hDUI, TDPageSubclassProc, kTDPageSubclassId);
+    SetWindowSubclassIfNeeded
+    (
+        hDUI,
+        TDPageSubclassProc,
+        kTDPageSubclassId,
+        [hDUI]()
+        {
+            // Change class background brush when subclassing.
+            const auto newBrush = reinterpret_cast<LONG_PTR>(
+                GetSolidBrush(TDDarkCol::kSecondary)
+            );
+            return ::SetClassLongPtr(hDUI, GCLP_HBRBACKGROUND, newBrush);
+        }
+    );
 
     d->found = true;
     return TRUE;