Fix rubberband selection outline position

Summary:
Before this patch, the outline of the rubberband selection control was down and to the right half a pixel too far.
This wasn't visible at 1x scaling because of the lack of antialiasing, but resulted in the top/left being too thin and bottom/right being too thick on high DPI displays.
To fix the issue, I enabled antialiasing and moved all sides inwards by half a pixel so that the outline would be pixel perfect.
I also added a function to get a QRectF with the correct size and position for rectangles with pen strokes.

Test Plan:
- Open dolphin and make a selection with the mouse
- Go to a Folder View Plasma widget and make a selection with the mouse

2x scaling
before: {F7818498, size=full}
after: {F7818500, size=full}

3x scaling
before: {F7818484}
after: {F7818483}

Reviewers: #vdg, #breeze, #plasma, hpereiradacosta, broulik

Reviewed By: hpereiradacosta

Subscribers: broulik, plasma-devel

Tags: #plasma, #breeze, #vdg

Differential Revision: https://phabricator.kde.org/D26001

kstyle/breezehelper.cpp

@@ -1436,10 +1436,29 @@ namespace Breeze
         #endif
     }
 
+    //______________________________________________________________________________
+    QRectF Helper::strokedRect( const QRect &rect, const int penWidth ) const
+    {
+        /* With a pen stroke width of 1, the rectangle should have each of its
+         * sides moved inwards by half a pixel. This allows the stroke to be
+         * pixel perfect instead of blurry from sitting between pixels and
+         * prevents the rectangle with a stroke from becoming larger than the
+         * original size of the rectangle.
+         */
+        qreal adjustment = 0.5 * penWidth;
+        return QRectF( rect ).adjusted( adjustment, adjustment, -adjustment, -adjustment );
+    }
+    
+    QRectF Helper::strokedRect( const QRectF &rect, const int penWidth ) const
+    {
+        qreal adjustment = 0.5 * penWidth;
+        return rect.adjusted( adjustment, adjustment, -adjustment, -adjustment );
+    }
+    
     //______________________________________________________________________________
     QRectF Helper::shadowRect( const QRectF& rect ) const
     { return rect.adjusted( 0.5, 0.5, -0.5, -0.5 ).translated( 0.5, 0.5 ); }
-
+    
     //______________________________________________________________________________
     QPainterPath Helper::roundedPath( const QRectF& rect, Corners corners, qreal radius ) const
     {

kstyle/breezehelper.h

@@ -307,6 +307,12 @@ namespace Breeze
         //* frame radius
         qreal frameRadius( qreal bias = 0 ) const
         { return qMax( qreal( Metrics::Frame_FrameRadius ) - 0.5 + bias, 0.0 ); }
+        
+        //* return a QRectF with the appropriate size for a rectangle with a pen stroke
+        QRectF strokedRect( const QRect &rect, const int penWidth = 1 ) const;
+        
+        //* return a QRectF with the appropriate size for a rectangle with a pen stroke
+        QRectF strokedRect( const QRectF &rect, const int penWidth = 1 ) const;
 
         protected:
 

kstyle/breezestyle.cpp

@@ -5388,15 +5388,19 @@ namespace Breeze
     bool Style::drawRubberBandControl( const QStyleOption* option, QPainter* painter, const QWidget* ) const
     {
 
-        const auto& palette( option->palette );
-        const auto rect( option->rect );
+        painter->save();
 
+        painter->setRenderHints( QPainter::Antialiasing );
+        const auto& palette( option->palette );
         auto color = palette.color( QPalette::Highlight );
-        painter->setPen( KColorUtils::mix( color, palette.color( QPalette::Active, QPalette::WindowText ) ) );
+        QPen pen = KColorUtils::mix( color, palette.color( QPalette::Active, QPalette::WindowText ) );
+        pen.setJoinStyle(Qt::RoundJoin);
+        painter->setPen( pen );
         color.setAlpha( 50 );
         painter->setBrush( color );
-        painter->setClipRegion( rect );
-        painter->drawRect( rect.adjusted( 0, 0, -1, -1 ) );
+        painter->drawRect( _helper->strokedRect( option->rect ) );
+
+        painter->restore();
         return true;
 
     }