QmlDesigner: Enable transient ScrollBars for StudioStyle

- Transient scroll bar is available for StudioStyle.
- Some changes are applied to slider style, because scrollBar and
sliders have some common usages within the style.

Task-number: QDS-9283
Change-Id: I7a8b7997cf4d20142a0524c4a071b93dfd06321d
Reviewed-by: Henning Gründl <henning.gruendl@qt.io>

src/libs/utils/transientscroll.cpp

@@ -0,0 +1,215 @@
+// Copyright (C) 2023 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0+ OR GPL-3.0 WITH Qt-GPL-exception-1.0
+
+#include "transientscroll.h"
+
+#include <QAbstractScrollArea>
+#include <QMouseEvent>
+#include <QPointer>
+#include <QStyle>
+#include <QStyleOption>
+
+using namespace Utils;
+
+static constexpr char transientScrollAreaSupportName[] = "transientScrollAreSupport";
+
+class Utils::ScrollAreaPrivate
+{
+public:
+    ScrollAreaPrivate(QAbstractScrollArea *area)
+        : area(area)
+    {
+        verticalScrollBar = new ScrollBar(area);
+        area->setVerticalScrollBar(verticalScrollBar);
+
+        horizontalScrollBar = new ScrollBar(area);
+        area->setHorizontalScrollBar(horizontalScrollBar);
+
+        area->setVerticalScrollBarPolicy(Qt::ScrollBarAsNeeded);
+        area->setHorizontalScrollBarPolicy(Qt::ScrollBarAsNeeded);
+    }
+
+    inline QRect scrollBarRect(ScrollBar *scrollBar)
+    {
+        QRect rect = viewPort ? viewPort->rect() : area->rect();
+        if (scrollBar->orientation() == Qt::Vertical) {
+            int mDiff = rect.width() - scrollBar->sizeHint().width();
+            return rect.adjusted(mDiff, 0, mDiff, 0);
+        } else {
+            int mDiff = rect.height() - scrollBar->sizeHint().height();
+            return rect.adjusted(0, mDiff, 0, mDiff);
+        }
+    }
+
+    inline void checkToFlashScroll(QPointer<ScrollBar> scrollBar, const QPoint &pos)
+    {
+        if (scrollBar.isNull())
+            return;
+
+        if (!scrollBar->style()->styleHint(
+                QStyle::SH_ScrollBar_Transient,
+                nullptr, scrollBar))
+            return;
+
+        if (scrollBarRect(scrollBar).contains(pos))
+            scrollBar->flash();
+    }
+
+    inline void checkToFlashScroll(const QPoint &pos)
+    {
+        checkToFlashScroll(verticalScrollBar, pos);
+        checkToFlashScroll(horizontalScrollBar, pos);
+    }
+
+    inline void installViewPort(QObject *eventHandler) {
+        QWidget *viewPort = area->viewport();
+        if (viewPort
+            && viewPort != this->viewPort
+            && viewPort->style()->styleHint(QStyle::SH_ScrollBar_Transient, nullptr, viewPort)
+            && (area->verticalScrollBarPolicy() != Qt::ScrollBarAlwaysOff
+                || area->horizontalScrollBarPolicy() != Qt::ScrollBarAlwaysOff)) {
+            viewPort->installEventFilter(eventHandler);
+            this->viewPort = viewPort;
+        }
+    }
+
+    inline void uninstallViewPort(QObject *eventHandler) {
+        if (viewPort) {
+            viewPort->removeEventFilter(eventHandler);
+            this->viewPort = nullptr;
+        }
+    }
+
+    QAbstractScrollArea *area = nullptr;
+    QPointer<QWidget> viewPort = nullptr;
+    QPointer<ScrollBar> verticalScrollBar;
+    QPointer<ScrollBar> horizontalScrollBar;
+};
+
+TransientScrollAreaSupport::TransientScrollAreaSupport(QAbstractScrollArea *scrollArea)
+    : QObject(scrollArea)
+    , d(new ScrollAreaPrivate(scrollArea))
+{
+    scrollArea->installEventFilter(this);
+}
+
+void TransientScrollAreaSupport::support(QAbstractScrollArea *scrollArea)
+{
+    QObject *prevSupport = scrollArea->property(transientScrollAreaSupportName)
+                               .value<QObject *>();
+    if (!prevSupport)
+        scrollArea->setProperty(transientScrollAreaSupportName,
+                                QVariant::fromValue(
+                                    new TransientScrollAreaSupport(scrollArea))
+                                );
+}
+
+TransientScrollAreaSupport::~TransientScrollAreaSupport()
+{
+    delete d;
+}
+
+bool TransientScrollAreaSupport::eventFilter(QObject *watched, QEvent *event)
+{
+    switch (event->type()) {
+    case QEvent::Enter: {
+        if (watched == d->area)
+            d->installViewPort(this);
+    }
+    break;
+    case QEvent::Leave: {
+        if (watched == d->area)
+            d->uninstallViewPort(this);
+    }
+    break;
+    case QEvent::MouseMove: {
+        if (watched == d->viewPort){
+            QMouseEvent *mouseEvent = static_cast<QMouseEvent *>(event);
+            if (mouseEvent) {
+                d->checkToFlashScroll(mouseEvent->pos());
+                return true;
+            }
+        }
+    }
+    break;
+    default:
+        break;
+    }
+    return QObject::eventFilter(watched, event);
+}
+
+class Utils::ScrollBarPrivate {
+public:
+    bool flashed = false;
+    int flashTimer = 0;
+};
+
+ScrollBar::ScrollBar(QWidget *parent)
+    : QScrollBar(parent)
+    , d(new ScrollBarPrivate)
+{
+}
+
+ScrollBar::~ScrollBar()
+{
+    delete d;
+}
+
+QSize ScrollBar::sizeHint() const
+{
+    QSize sh = QScrollBar::sizeHint();
+    if (style()->styleHint(QStyle::SH_ScrollBar_Transient, nullptr, this)) {
+        constexpr int thickness = 10;
+        if (orientation() == Qt::Horizontal)
+            sh.setHeight(thickness);
+        else
+            sh.setWidth(thickness);
+    } else {
+        constexpr int thickness = 12;
+        if (orientation() == Qt::Horizontal)
+            sh.setHeight(thickness);
+        else
+            sh.setWidth(thickness);
+    }
+    return sh;
+}
+
+void ScrollBar::flash()
+{
+    if (!d->flashed && style()->styleHint(QStyle::SH_ScrollBar_Transient, nullptr, this)) {
+        d->flashed = true;
+        if (!isVisible())
+            show();
+        else
+            update();
+    }
+    if (!d->flashTimer)
+        d->flashTimer = startTimer(0);
+}
+
+void ScrollBar::initStyleOption(QStyleOptionSlider *option) const
+{
+    QScrollBar::initStyleOption(option);
+
+    if (d->flashed && style()->styleHint(QStyle::SH_ScrollBar_Transient, nullptr, this))
+        option->state |= QStyle::State_On;
+}
+
+bool ScrollBar::event(QEvent *event)
+{
+    switch (event->type()) {
+    case QEvent::Timer:
+        if (static_cast<QTimerEvent *>(event)->timerId() == d->flashTimer) {
+            if (d->flashed && style()->styleHint(QStyle::SH_ScrollBar_Transient, nullptr, this)) {
+                d->flashed = false;
+                update();
+            }
+            killTimer(d->flashTimer);
+            d->flashTimer = 0;
+        }
+        break;
+    default:
+        break;
+    }
+    return QScrollBar::event(event);
+}