qtabwidget_cornerwidget_stretch.py

import sys

import PyQt5.QtWidgets


class ProxyStyle(PyQt5.QtWidgets.QProxyStyle):
    def subElementRect(self, sr, opt, widget):
        r = super().subElementRect(sr, opt, widget)
        if sr == PyQt5.QtWidgets.QStyle.SE_TabWidgetRightCorner:
            twf = opt
            paneRect = self.subElementRect(PyQt5.QtWidgets.QStyle.SE_TabWidgetTabPane, twf, widget)
            if twf.shape in (PyQt5.QtWidgets.QTabBar.RoundedNorth, PyQt5.QtWidgets.QTabBar.TriangularNorth):
                # r = PyQt5.QtCore.QRect(PyQt5.QtCore.QPoint(paneRect.width() - twf.rightCornerWidgetSize.width(),
                #                                            paneRect.y() - twf.rightCornerWidgetSize.height()),
                #                        twf.rightCornerWidgetSize)
                width = paneRect.width() - twf.tabBarSize.width()
                width = max(width, twf.rightCornerWidgetSize.width())
                r = PyQt5.QtCore.QRect(PyQt5.QtCore.QPoint(paneRect.width() - width,
                                                           paneRect.y() - twf.rightCornerWidgetSize.height()),
                                       PyQt5.QtCore.QSize(width, twf.rightCornerWidgetSize.height()))
            elif twf.shape in (PyQt5.QtWidgets.QTabBar.RoundedSouth, PyQt5.QtWidgets.QTabBar.TriangularSouth):
                r = PyQt5.QtCore.QRect(PyQt5.QtCore.QPoint(paneRect.width() - twf.rightCornerWidgetSize.width(),
                                                           paneRect.height(),
                                       twf.rightCornerWidgetSize.height()))

            r = self.visualRect(twf.direction, twf.rect, r)

        return r
                
class ProxyStyle2(PyQt5.QtWidgets.QProxyStyle):
    def subElementRect(self, sr, opt, widget):
        r = super().subElementRect(sr, opt, widget)
        if sr == PyQt5.QtWidgets.QStyle.SE_TabWidgetRightCorner and widget.property('expand_corner_widget'):
            paneRect = self.subElementRect(PyQt5.QtWidgets.QStyle.SE_TabWidgetTabPane, opt, widget)
            width = paneRect.width() - opt.tabBarSize.width()
            width = max(width, opt.rightCornerWidgetSize.width())
            delta_width = width - r.width()
            r.adjust(-delta_width, 0, delta_width, 0)

        return r

app = PyQt5.QtWidgets.QApplication(sys.argv)
app.setStyle(ProxyStyle2())

tw = PyQt5.QtWidgets.QTabWidget()
tw.setProperty('expand_corner_widget', True)
for name in ('red', 'green', 'blue'):
    w = PyQt5.QtWidgets.QLabel(name)
    tw.addTab(w, name)

a = PyQt5.QtWidgets.QPushButton('a')
b = PyQt5.QtWidgets.QPushButton('b')
tb = PyQt5.QtWidgets.QToolBar()
tb.addWidget(a)
tb.addWidget(b)
tw.setCornerWidget(tb)

window = PyQt5.QtWidgets.QMainWindow()
window.setCentralWidget(tw)

window.show()

app.exec()