Improve preview frame visibilty of cursor condition

* Add transparent diagonal stripes to frame
* Do not show frame if invalid selection was made

src/utils/striped-frame.cpp

@@ -0,0 +1,37 @@
+#include "striped-frame.hpp"
+
+#include <cmath>
+
+constexpr QColor stripeColor = QColor(255, 255, 255, 50);
+constexpr int stripeWidth = 30;
+constexpr int stripeSpacing = 30;
+constexpr qreal sqrtOf2 = 1.41421356237309;
+constexpr int painterVerticalOffset = stripeWidth / sqrtOf2 + 1;
+constexpr int rotatedPainterSpacing =
+	(stripeWidth + stripeSpacing) / sqrtOf2 + 1;
+constexpr qreal horizontalMoveStep = stripeWidth + stripeSpacing;
+constexpr qreal verticalMoveStep = -horizontalMoveStep;
+
+void StripedFrame::paintEvent(QPaintEvent *event)
+{
+	QFrame::paintEvent(event);
+
+	QPainter painter(this);
+	painter.setPen(Qt::NoPen);
+	painter.setBrush(stripeColor);
+
+	const int biggerSideSize = width() > height() ? width() : height();
+	const int numStripes = biggerSideSize / rotatedPainterSpacing + 1;
+
+	painter.translate(0, -painterVerticalOffset);
+	painter.rotate(45);
+
+	const qreal diagonalLength =
+		std::sqrt(width() * width() + height() * height());
+	const qreal stripeLength = diagonalLength * 2;
+
+	for (int i = 0; i < numStripes; ++i) {
+		painter.drawRect(0, 0, stripeWidth, stripeLength);
+		painter.translate(horizontalMoveStep, verticalMoveStep);
+	}
+}

src/utils/striped-frame.hpp

@@ -0,0 +1,11 @@
+#pragma once
+#include <QFrame>
+#include <QPainter>
+
+class StripedFrame : public QFrame {
+public:
+	StripedFrame(QWidget *parent = nullptr) : QFrame(parent) {}
+
+protected:
+	void paintEvent(QPaintEvent *event) override;
+};