fix: check QPainter::isActive() before drawing in Wayland (flameshot-org#4496)

On Wayland, the backing store may not be ready when paintEvent is called,
causing QPainter to fail with 'Paint device returned engine == 0'.

This adds isActive() checks to prevent crashes when the painter
cannot be initialized.

Fixes crashes on KDE Plasma Wayland with Qt 6.

Author: benignobjunior

src/widgets/capture/capturewidget.cpp

@@ -644,6 +644,9 @@ void CaptureWidget::paintEvent(QPaintEvent* paintEvent)
 {
     Q_UNUSED(paintEvent)
     QPainter painter(this);
+    if (!painter.isActive()) {
+        return;
+    }
     GeneralConf::xywh_position position =
       static_cast<GeneralConf::xywh_position>(m_config.showSelectionGeometry());
     /* QPainter::save and restore is somewhat costly so we try to guess

src/widgets/capture/magnifierwidget.cpp

@@ -35,6 +35,9 @@ MagnifierWidget::MagnifierWidget(const QPixmap& p,
 void MagnifierWidget::paintEvent(QPaintEvent*)
 {
     QPainter p(this);
+    if (!p.isActive()) {
+        return;
+    }
     if (m_square) {
         drawMagnifier(p);
     } else {

src/widgets/capture/notifierbox.cpp

@@ -34,6 +34,9 @@ void NotifierBox::enterEvent(QEnterEvent*)
 void NotifierBox::paintEvent(QPaintEvent*)
 {
     QPainter painter(this);
+    if (!painter.isActive()) {
+        return;
+    }
     // draw Ellipse
     painter.setRenderHint(QPainter::Antialiasing);
     painter.setBrush(QBrush(m_bgColor, Qt::SolidPattern));

src/widgets/capture/overlaymessage.cpp

@@ -99,6 +99,8 @@ QString OverlayMessage::compileFromKeyMap(
 void OverlayMessage::paintEvent(QPaintEvent* e)
 {
     QPainter painter(this);
+    if (!painter.isActive())
+        return;
     painter.setRenderHint(QPainter::Antialiasing);
 
     painter.setBrush(QBrush(m_fillColor, Qt::SolidPattern));

src/widgets/capture/selectionwidget.cpp

@@ -380,6 +380,9 @@ void SelectionWidget::parentMouseMoveEvent(QMouseEvent* e)
 void SelectionWidget::paintEvent(QPaintEvent*)
 {
     QPainter p(this);
+    if (!p.isActive()) {
+        return;
+    }
     p.setPen(m_color);
     p.drawRect(rect() + QMargins(0, 0, -1, -1));
     p.setRenderHint(QPainter::Antialiasing);

src/widgets/colorpickerwidget.cpp

@@ -28,6 +28,8 @@ const QVector<QColor>& ColorPickerWidget::getDefaultLargeColorPalette()
 void ColorPickerWidget::paintEvent(QPaintEvent* e)
 {
     QPainter painter(this);
+    if (!painter.isActive())
+        return;
     painter.setRenderHint(QPainter::Antialiasing);
     painter.setPen(QColor(Qt::black));
 

src/widgets/loadspinner.cpp

@@ -54,6 +54,8 @@ void LoadSpinner::stop()
 void LoadSpinner::paintEvent(QPaintEvent*)
 {
     QPainter painter(this);
+    if (!painter.isActive())
+        return;
     painter.setRenderHint(QPainter::Antialiasing, true);
     auto pen = QPen(m_color);
 

src/widgets/orientablepushbutton.cpp

@@ -38,6 +38,8 @@ void OrientablePushButton::paintEvent(QPaintEvent* event)
     Q_UNUSED(event)
 
     QStylePainter painter(this);
+    if (!painter.isActive())
+        return;
     QStyleOptionButton option;
     initStyleOption(&option);
 

src/widgets/panel/colorgrabwidget.cpp

@@ -142,6 +142,8 @@ bool ColorGrabWidget::eventFilter(QObject*, QEvent* event)
 void ColorGrabWidget::paintEvent(QPaintEvent*)
 {
     QPainter painter(this);
+    if (!painter.isActive())
+        return;
     painter.drawImage(QRectF(0, 0, width(), height()), m_previewImage);
 }