Update viewport margins.

def _update_viewport_margins(self):
        """Update viewport margins."""
        top = 0
        left = 0
        right = 0
        bottom = 0
        for panel in self.panels_for_zone(Panel.Position.LEFT):
            if panel.isVisible():
                width = panel.sizeHint().width()
                left += width
        for panel in self.panels_for_zone(Panel.Position.RIGHT):
            if panel.isVisible():
                width = panel.sizeHint().width()
                right += width
        for panel in self.panels_for_zone(Panel.Position.TOP):
            if panel.isVisible():
                height = panel.sizeHint().height()
                top += height
        for panel in self.panels_for_zone(Panel.Position.BOTTOM):
            if panel.isVisible():
                height = panel.sizeHint().height()
                bottom += height
        self._margin_sizes = (top, left, right, bottom)
        self.editor.setViewportMargins(left, top, right, bottom)