Mitre pause-border corners to avoid double-darkening

Clip each side of the pause vignette to a trapezoid so the four gradient
strips meet along the corner diagonals instead of overlapping, which was
compositing two 50% blacks into darker, oddly-shaded corners.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc

src/ui/GameWorldView.cpp

@@ -19,6 +19,7 @@
 #include <QMessageBox>
 #include <QMouseEvent>
 #include <QPainter>
+#include <QPainterPath>
 #include <QPen>
 #include <QPolygonF>
 #include <QRadialGradient>
@@ -1709,29 +1710,50 @@ void GameWorldView::drawPauseBorder(QPainter& painter)
     const QColor edge(0, 0, 0, 128);        // 50% black at the viewport edge
     const QColor inner(0, 0, 0, 0);         // fully transparent toward the center
 
+    const qreal w = width();
+    const qreal h = height();
+
     painter.save();
 
     // Each side is a linear gradient fading from `edge` at the viewport border to
-    // `inner` toward the center. Corners overlap and read as a natural vignette.
+    // `inner` toward the center. The four sides are clipped to trapezoids that meet
+    // along the corner diagonals (a mitred picture frame), so the strips never
+    // overlap and don't double-darken the corners. Because the border thickness is
+    // the same on every side, both adjoining gradients evaluate equally along each
+    // 45-degree diagonal and join seamlessly.
+    const std::function<void(const QLinearGradient&, const QPolygonF&)> drawSide =
+        [&](const QLinearGradient& gradient, const QPolygonF& trapezoid) {
+        QPainterPath clip;
+        clip.addPolygon(trapezoid);
+        painter.save();
+        painter.setClipPath(clip);
+        painter.fillRect(QRectF(0, 0, w, h), gradient);
+        painter.restore();
+    };
+
     QLinearGradient left(0, 0, borderPx, 0);
     left.setColorAt(0.0, edge);
     left.setColorAt(1.0, inner);
-    painter.fillRect(QRectF(0, 0, borderPx, height()), left);
+    drawSide(left, QPolygonF({ QPointF(0, 0), QPointF(borderPx, borderPx),
+                              QPointF(borderPx, h - borderPx), QPointF(0, h) }));
 
-    QLinearGradient right(width(), 0, width() - borderPx, 0);
+    QLinearGradient right(w, 0, w - borderPx, 0);
     right.setColorAt(0.0, edge);
     right.setColorAt(1.0, inner);
-    painter.fillRect(QRectF(width() - borderPx, 0, borderPx, height()), right);
+    drawSide(right, QPolygonF({ QPointF(w, 0), QPointF(w - borderPx, borderPx),
+                               QPointF(w - borderPx, h - borderPx), QPointF(w, h) }));
 
     QLinearGradient top(0, 0, 0, borderPx);
     top.setColorAt(0.0, edge);
     top.setColorAt(1.0, inner);
-    painter.fillRect(QRectF(0, 0, width(), borderPx), top);
+    drawSide(top, QPolygonF({ QPointF(0, 0), QPointF(w, 0),
+                             QPointF(w - borderPx, borderPx), QPointF(borderPx, borderPx) }));
 
-    QLinearGradient bottom(0, height(), 0, height() - borderPx);
+    QLinearGradient bottom(0, h, 0, h - borderPx);
     bottom.setColorAt(0.0, edge);
     bottom.setColorAt(1.0, inner);
-    painter.fillRect(QRectF(0, height() - borderPx, width(), borderPx), bottom);
+    drawSide(bottom, QPolygonF({ QPointF(0, h), QPointF(w, h),
+                                QPointF(w - borderPx, h - borderPx), QPointF(borderPx, h - borderPx) }));
 
     painter.restore();
 }