Horizontal slider set back to 0 after switching application windows

[schneider42]

Whenever I open a file in Inspectrum and perform one of the following actions, the horizontal slider is set back to 0 (i.e. the leftmost position):

• Switch to another full screen window
• Change to another virtual desktop

Similar to #170 I've collected a log of what happens to the application:

PlotView::viewportEvent  QEvent::Type(WindowDeactivate)
PlotView::viewportEvent  QEvent::Type(Hide)
PlotView::viewportEvent  QEvent::Type(Show)
PlotView::updateView
PlotView::viewportEvent  QEvent::Type(Paint)
PlotView::paintEvent  QEvent::Type(Paint)
PlotView::viewportEvent  QEvent::Type(WindowActivate)
PlotView::updateView
PlotView::viewportEvent  QEvent::Type(Paint)
PlotView::paintEvent  QEvent::Type(Paint)

[schneider42]

As I'm using xmonad as my window manager I got concerned that this is might be an issue with my setup. I tried again with the standard GNOME 3 that comes with Ubuntu 18.04 and 20.04.

On GNOME 3 I was able to reproduce this issue by minimizing the window to the task bar and then restoring it again.

I dug a bit deeper in to the chain of events and this is what's happening:

• User asks window manager to hide/minimize Inspectrum
• PlotView::viewportEvent() gets called with QEvent::Type(WindowDeactivate)
• PlotView::viewportEvent() gets called with QEvent::Type(Leave)
• PlotView::viewportEvent() gets called with QEvent::Type(Hide)
• User asks window manager to show/restore Inspectrum
• PlotView::viewportEvent() gets called with QEvent::Type(Show)
• PlotView::scrollContentsBy() gets called with dx set to the amount the scroll bar was moved before hiding/minimizing the window.
  • At this point both horizontalScrollBar()->value() and horizontalScrollBar()->maximum() evaluate to 0. It looks like something internal to Qt changed the scrollbar between the calls to PlotView::viewportEvent() and PlotView::scrollContentsBy().
• PlotView::updateView() gets called by PlotView::scrollContentsBy(). It resets the maximum of the scrollbar back to file length - window size. The original value of the scrollbar is lost though.

After discovering this I had a look at what QGraphicsView (a parent class of PlotView) offers but found nothing which I figured would immediately solve the issue. It looks like QGraphicsView is used a bit outside its original intention as it expects a QGraphicsScene as its content?

I thought that one function might be interesting to get more insight into the issue and I implemented PlotView::showEvent() simply for debugging purposes. Thing is: that solved the issue.

Simply implementing an empty PlotView::showEvent() was enough to fix the issue on xmonad and GNOME 3.