QProxyStyle::ensureBaseStyle() should always reset the proxy of its base style.

Here is an example why:

QProxyStyle* proxyWidgetA = new QProxyStyle(base);
proxyWidgetA->standardPalette(); // call QProxyStyle::ensureBaseStyle()
Q_ASSERT(base->proxy() == proxyWidgetA);

QProxyStyle* proxyWidgetB = new QProxyStyle(base);
proxyWidgetB->standardPalette(); // QProxyStyle::ensureBaseStyle()
Q_ASSERT(base->proxy() == proxyWidgetB);

// When the widget A is drawn...
proxyWidgetA->standardPalette(); // QProxyStyle::ensureBaseStyle()
Q_ASSERT(base->proxy() == proxyWidgetA); // -> fail

Please also note that QProxyStyle::QProxyStyle() QProxyStyle::setBaseStyle() and QProxyStyle::ensureBaseStyle() could benefit from refactorization. (calls to setProxy(...) and setParent(...) are duplicated). Worst, QProxyStyle::QProxyStyle(QStyle* s) creates an unnecessary style because of the order of the calls (in constructor, setParent() triggers a ensureBaseStyle() that creates a system style)