I'm trying to implement a colorpicker application using Qt6. I have a custom window class (which will be the main application window) which is a subclass of QFrame and I want it to have round corners. Github link
Link to the whole code base is here

1. Setting border-radius in stylesheets alone did not work. I get round corners but there's a rectangular corner underneath the round corner, which defeats the purpose.

2. Overriding the paintEvent method gave me what I wanted (I had to use setAttribute(Qt::WA_TranslucentBackground) to avoid getting white rectangular corners underneath the rounded corners here) .

Even though this works, II'm worried about the performance of my implementation. Here's the method implementation.

virtual inline void paintEvent(QPaintEvent* _paint_event) noexcept override {
            QPainter _painter { this };
            _painter.setRenderHint(QPainter::RenderHint::Antialiasing);
            _painter.setBrush(
                QBrush {
                   QColor {
                           _slider_values[configs::rgb_offsets::RED], _slider_values[configs::rgb_offsets::GREEN], _slider_values[configs::rgb_offsets::BLUE] }
            }
            );
            _painter.setPen(Qt::GlobalColor::transparent); // thin borders

            QRect _current_rect { rect() };
            _painter.drawRoundedRect(_current_rect, 10, 0);
            QWidget::paintEvent(_paint_event);
            update(); // without this only the edges of the sliders got painted while the background wasn't responsive???
        }

Without the call to update() at the end of paintEvent(), this is what I got.

Prior to overriding the paintEvent method, I used the setPalette method to update the background with the current slider values.

        [[maybe_unused]] inline void __attribute__((__always_inline__)) __update_bg() noexcept {
            // update the colour palette with the current state of the sliders
            _palette.setColor(
                QPalette::Window,
                QColor { _slider_values[configs::rgb_offsets::RED], _slider_values[configs::rgb_offsets::GREEN], _slider_values[configs::rgb_offsets::BLUE] }
            );
            setPalette(_palette); // set the updated palette, triggering a window redraw
        }

My questions are;

1. Am I doing this in the most efficient way? (I'm not looking for an idiomatic way, I'm more interested performance).
2. What is the best way to implement the paintEvent method, granted that I only use it for round corners and updating the background colour based on the slider positions?
3. What's the best way to introduce a custom title bar at the top of this window so it looks like the following image. (i.e. a custom title bar with custom icons)
   

Thank you in advance. I'd appreciate any help regarding this.

System info:
Qt version 6.10.2 on Fedora Linux 43 (Workstation Edition)