[LyX/2.2.x] Fix "scroll here" feature of scrollbar (#10311)

[Guillaume Munch]

commit 49ce94f636ba3d6513724fd7b00969cd39f596eb
Author: Guillaume Munch <g...@lyx.org>
Date:   Fri Jul 29 21:27:13 2016 +0100

    Fix "scroll here" feature of scrollbar (#10311)
    
    Prevent setRange() from causing a recursive call to scrollTo(). Reduces 
three
    calls of scrollTo() to one call for all scrolling functions of the scroll 
bar
    (e.g. clicking on the arrow, dragging, or clicking somewhere on the 
scrollbar).
    
    (cherry picked from commit 4d7b912ca1890b5b0b3ed29568b7091a7e7baef0)
---
 src/BufferView.cpp                |    6 ++++++
 src/frontends/qt4/GuiWorkArea.cpp |    7 +++----
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/src/BufferView.cpp b/src/BufferView.cpp
index f304798..78abc87 100644
--- a/src/BufferView.cpp
+++ b/src/BufferView.cpp
@@ -565,6 +565,12 @@ void BufferView::updateScrollbar()
                d->scrollbarParameters_.max -= minVisiblePart();
        else
                d->scrollbarParameters_.max -= 
d->scrollbarParameters_.page_step;
+
+       // 0 must be inside the range as it denotes the current position
+       if (d->scrollbarParameters_.max < 0)
+               d->scrollbarParameters_.max = 0;
+       if (d->scrollbarParameters_.min > 0)
+               d->scrollbarParameters_.min = 0;
 }
 
 
diff --git a/src/frontends/qt4/GuiWorkArea.cpp 
b/src/frontends/qt4/GuiWorkArea.cpp
index 7cb732e..7b8b088 100644
--- a/src/frontends/qt4/GuiWorkArea.cpp
+++ b/src/frontends/qt4/GuiWorkArea.cpp
@@ -675,10 +675,9 @@ void GuiWorkArea::toggleCursor()
 void GuiWorkArea::Private::updateScrollbar()
 {
        ScrollbarParameters const & scroll_ = 
buffer_view_->scrollbarParameters();
-       // WARNING: don't touch at the scrollbar value like this:
-       //   verticalScrollBar()->setValue(scroll_.position);
-       // because this would cause a recursive signal/slot calling with
-       // GuiWorkArea::scrollTo
+       // Block signals to prevent setRange() and setSliderPosition from 
causing
+       // recursive calls via the signal valueChanged. (#10311)
+       QSignalBlocker blocker(p->verticalScrollBar());
        p->verticalScrollBar()->setRange(scroll_.min, scroll_.max);
        p->verticalScrollBar()->setPageStep(scroll_.page_step);
        p->verticalScrollBar()->setSingleStep(scroll_.single_step);