D27110: Fix scrollbar sometimes reappearing when set to 'hidden' and scrolling using the touchpad

[Ferdinand Bachmann]

yrlf added a comment.


  I have looked at this a bit more today, in order to help people reproduce this:
  
  - the bug only appears on Wayland, not on X11 or XWayland
  
  when looking at the differences between the QWheelEvents between X11 and Wayland, I noticed that the Qt::ScrollPhase of the events is different!
  
  - Mouse scrolling on Wayland: phase is always 0 (phase not supported)
  - Touchpad scrolling on Wayland: phase is 1, then 2, then 3 (beginscroll, updatescroll, endscroll)
  - Mouse scrolling on X11: phase is always 0
  - Touchpad scrolling on X11: phase is always 0
  
  This is most likely what triggers the scrollbar becoming visible, because of this code in `qscrollbar.cpp` in Qt 5.14
  
    void QScrollBar::wheelEvent(QWheelEvent *event)
    {
        event->ignore();
        bool horizontal = qAbs(event->angleDelta().x()) > qAbs(event->angleDelta().y());
        // The vertical wheel can be used to scroll a horizontal scrollbar, but only if
        // there is no simultaneous horizontal wheel movement.  This is to avoid chaotic
        // scrolling on touchpads.
        if (!horizontal && event->angleDelta().x() != 0 && orientation() == Qt::Horizontal)
            return;
        // scrollbar is a special case - in vertical mode it reaches minimum
        // value in the upper position, however QSlider's minimum value is on
        // the bottom. So we need to invert the value, but since the scrollbar is
        // inverted by default, we need to invert the delta value only for the
        // horizontal orientation.
        int delta = horizontal ? -event->angleDelta().x() : event->angleDelta().y();
        Q_D(QScrollBar);
        if (d->scrollByDelta(horizontal ? Qt::Horizontal : Qt::Vertical, event->modifiers(), delta))
            event->accept();
    
        if (event->phase() == Qt::ScrollBegin)
            d->setTransient(false);
        else if (event->phase() == Qt::ScrollEnd)
            d->setTransient(true);
    }
  
  note the check of `event->phase()` near the bottom. `d->setTransient(false)` subsequently calls `q-show()`, which makes the scrollbar visible.

REPOSITORY
  R319 Konsole

REVISION DETAIL
  https://phabricator.kde.org/D27110

To: yrlf, #konsole, hindenburg, tcanabrava
Cc: konsole-devel, cent, EspadaV8, fbampaloukas, thsurrel, ngraham, maximilianocuria, hindenburg
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