QFusionStyle: fix the checkbox rendering in low DPI settings

Fixed some regression of checkbox's size brought in by c9f68a5.

Using qreal and QRectF to avoid the round error accumulation
of sizes in low dpi situations.

Tweaked the look of the check mark.

Task-number: QTBUG-66343
Change-Id: Iad84f2f4503b53faf078ad52e13b6fbe88267426
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
(cherry picked from commit ea21b36836abd6c2bb6d139c9d4b8149acdc4785)
Reviewed-by: Liang Qi <liang.qi@qt.io>

src/widgets/styles/qfusionstyle.cpp

@@ -786,7 +786,7 @@ void QFusionStyle::drawPrimitive(PrimitiveElement elem,
             painter->drawRect(rect);
 
             QColor checkMarkColor = option->palette.text().color().darker(120);
-            const int checkMarkPadding = 1 + rect.width() * 0.2; // at least one pixel padding
+            const qreal checkMarkPadding = 1 + rect.width() * 0.13; // at least one pixel padding
 
             if (checkbox->state & State_NoChange) {
                 gradient = QLinearGradient(rect.topLeft(), rect.bottomLeft());
@@ -798,20 +798,23 @@ void QFusionStyle::drawPrimitive(PrimitiveElement elem,
                 painter->setPen(QPen(checkMarkColor, 1));
                 painter->setBrush(gradient);
                 painter->drawRect(rect.adjusted(checkMarkPadding, checkMarkPadding, -checkMarkPadding, -checkMarkPadding));
+
             } else if (checkbox->state & State_On) {
-                qreal penWidth = QStyleHelper::dpiScaled(1.8);
-                penWidth = qMax(penWidth , 0.18 * rect.height());
-                penWidth = qMin(penWidth , 0.30 * rect.height());
+                qreal penWidth = QStyleHelper::dpiScaled(1.5);
+                penWidth = qMax(penWidth , 0.13 * rect.height());
+                penWidth = qMin(penWidth , 0.20 * rect.height());
+                QPen checkPen = QPen(checkMarkColor, penWidth);
                 checkMarkColor.setAlpha(210);
-                painter->setPen(QPen(checkMarkColor, penWidth));
+                painter->translate(dpiScaled(-0.8), dpiScaled(0.5));
+                painter->setPen(checkPen);
                 painter->setBrush(Qt::NoBrush);
-                painter->translate(-0.8, 0.5);
 
                 // Draw checkmark
                 QPainterPath path;
-                path.moveTo(1.33 * checkMarkPadding, rect.height() / 2.0);
-                path.lineTo(rect.width() / 2.0, rect.height() - checkMarkPadding);
-                path.lineTo(rect.width() - checkMarkPadding * 0.92, checkMarkPadding);
+                const qreal rectHeight = rect.height(); // assuming height equals width
+                path.moveTo(checkMarkPadding + rectHeight * 0.11, rectHeight * 0.47);
+                path.lineTo(rectHeight * 0.5, rectHeight - checkMarkPadding);
+                path.lineTo(rectHeight - checkMarkPadding, checkMarkPadding);
                 painter->drawPath(path.translated(rect.topLeft()));
             }
         }
@@ -1581,8 +1584,8 @@ void QFusionStyle::drawControl(ControlElement element, const QStyleOption *optio
 
             bool ignoreCheckMark = false;
             const int checkColHOffset = windowsItemHMargin + windowsItemFrame - 1;
-            int checkcol = qMax(menuItem->rect.height() * 0.7,
-                                      qMax(menuItem->maxIconWidth * 1.0, dpiScaled(17))); // icon checkbox's highlihgt column width
+            int checkcol = qMax(menuItem->rect.height() * 0.79,
+                                      qMax(menuItem->maxIconWidth * 1.0, dpiScaled(21))); // icon checkbox's highlihgt column width
             if (
 #if QT_CONFIG(combobox)
                 qobject_cast<const QComboBox*>(widget) ||
@@ -1591,10 +1594,12 @@ void QFusionStyle::drawControl(ControlElement element, const QStyleOption *optio
                 ignoreCheckMark = true; //ignore the checkmarks provided by the QComboMenuDelegate
 
             if (!ignoreCheckMark) {
-                // Check
-                const int boxMargin = dpiScaled(4);
-                const int boxWidth = checkcol - 2 * boxMargin;
-                QRect checkRect(option->rect.left() + boxMargin + checkColHOffset, option->rect.center().y() - boxWidth/2 + 1, boxWidth, boxWidth);
+                // Check, using qreal and QRectF to avoid error accumulation
+                const qreal boxMargin = dpiScaled(3.5);
+                const qreal boxWidth = checkcol - 2 * boxMargin;
+                QRectF checkRectF(option->rect.left() + boxMargin + checkColHOffset, option->rect.center().y() - boxWidth/2 + 1, boxWidth, boxWidth);
+                QRect checkRect = checkRectF.toRect();
+                checkRect.setWidth(checkRect.height()); // avoid .toRect() round error results in non-perfect square
                 checkRect = visualRect(menuItem->direction, menuItem->rect, checkRect);
                 if (checkable) {
                     if (menuItem->checkType & QStyleOptionMenuItem::Exclusive) {