even slimmer scrollbars

Summary:
the scrollbar handle will be very slim normally, and grow a bit on mouse
over, this makes it work better with QtQuickControls2 listviews where
the contents overlap the scrollbar (as other platform are going in
this direction since they all have scrollbars visible only on scroll
or mouse over)
this makes scrollbars appear more light but still always visible to
convey information

right now follows animations enabled to use this, as the scrollbar groove already did that, tough a "slim scrollbars" settings may make sense

Test Plan: works both as qwidget and with the qstyle-based qqc2-desktop-style

Reviewers: #plasma, #kirigami, #vdg, hpereiradacosta, abetts

Reviewed By: #plasma, #vdg, hpereiradacosta, abetts

Subscribers: davidedmundson, ngraham, colomar, abetts, plasma-devel

Tags: #plasma, #kirigami

Differential Revision: https://phabricator.kde.org/D9792

kstyle/breezestyle.cpp

@@ -5009,12 +5009,19 @@ namespace Breeze
         const auto& rect( option->rect );
         const auto& palette( option->palette );
 
+        //try to understand if anywhere the widget is under mouse, not just the handle, use _animations in case of QWidget, option->styleObject in case of QML
+        const bool widgetMouseOver( widget ? _animations->scrollBarEngine().isHovered( widget, QStyle::SC_ScrollBarGroove )  : option->styleObject->property("hover").toBool());
+
+        qreal grooveAnimationOpacity( _animations->scrollBarEngine().opacity( widget, QStyle::SC_ScrollBarGroove ) );
+        if( grooveAnimationOpacity == AnimationData::OpacityInvalid ) grooveAnimationOpacity = (widgetMouseOver ? 1 : 0);
+        const qreal handleSize = StyleConfigData::animationsEnabled() ? ((Metrics::ScrollBar_SliderWidth / 2.0) * (1 - grooveAnimationOpacity) + Metrics::ScrollBar_SliderWidth  * grooveAnimationOpacity) : (int)Metrics::ScrollBar_SliderWidth;
+
         // define handle rect
         QRect handleRect;
         const State& state( option->state );
         const bool horizontal( state & State_Horizontal );
-        if( horizontal ) handleRect = centerRect( rect, rect.width(), Metrics::ScrollBar_SliderWidth );
-        else handleRect = centerRect( rect, Metrics::ScrollBar_SliderWidth, rect.height() );
+        if( horizontal ) handleRect = centerRect( rect, rect.width(), handleSize );
+        else handleRect = centerRect( rect, handleSize, rect.height() );
 
         const bool enabled( state & State_Enabled );
         const bool mouseOver( enabled && ( state & State_MouseOver ) );
@@ -5031,7 +5038,10 @@ namespace Breeze
 
         const auto mode( _animations->scrollBarEngine().animationMode( widget, SC_ScrollBarSlider ) );
         const qreal opacity( _animations->scrollBarEngine().opacity( widget, SC_ScrollBarSlider ) );
-        const auto color = _helper->scrollBarHandleColor( palette, mouseOver, hasFocus, opacity, mode );
+        auto color = _helper->scrollBarHandleColor( palette, mouseOver, hasFocus, opacity, mode );
+        if (StyleConfigData::animationsEnabled()) {
+            color.setAlphaF(color.alphaF() * (0.7 + 0.3 * grooveAnimationOpacity));
+        }
 
         _helper->renderScrollBarHandle( painter, handleRect, color );
         return true;