[kstyle]: Don't attempt to adjust contentsMargins on QMainWindows

For most applications, this isn't necessary as they rarely draw in the top pixel,
and implementing this safely is problematic.

BUG: 432246

kstyle/breezetoolsareamanager.cpp

@@ -58,20 +58,12 @@ namespace Breeze {
         Q_ASSERT(window);
 
         int itemHeight = window->menuWidget() ? window->menuWidget()->height() : 0;
-        for (auto item : _windows[const_cast<QMainWindow*>(window)]) {
+        for (auto item : _windows[window]) {
             if (!item.isNull() && item->isVisible() && window->toolBarArea(item) == Qt::TopToolBarArea) {
                 itemHeight = qMax(item->mapTo(window, item->rect().bottomLeft()).y(), itemHeight);
             }
         }
 
-        if (itemHeight == 0 && !(window->property(PropertyNames::noSeparator).toBool()) && !window->isFullScreen()) {
-            auto win = const_cast<QMainWindow*>(window);
-            win->setContentsMargins(0, 1, 0, 0);
-        } else {
-            auto win = const_cast<QMainWindow*>(window);
-            win->setContentsMargins(0, 0, 0, 0);
-        }
-
         return QRect(0, 0, window->width(), itemHeight);
     }
 

kstyle/breezetoolsareamanager.h

@@ -31,7 +31,7 @@ namespace Breeze {
 
     private:
         Helper* _helper;
-        QHash<QMainWindow*,QVector<QPointer<QToolBar>>> _windows;
+        QHash<const QMainWindow*,QVector<QPointer<QToolBar>>> _windows;
         KSharedConfigPtr _config;
         KConfigWatcher::Ptr _watcher;
         QPalette _palette = QPalette();