QPixmapStyle: do not assume minimum == 0 when painting progress bars

The arithmetic used to calculate the size of the progress bar fill in
QPixmapStyle assumed minimum == 0, but that does not necessarily
hold, since the minimum value is user-defined.

So, fix the arithmetic to take the minimum into account, taking care,
as done elsewhere, to avoid signed integer and qreal=float overflows,
by using qint64 and double, respectively.

[ChangeLog][QtWidgets][QPixmapStyle] Now handles progress bars with
minimum != 0 correctly.

Change-Id: I738ded56e8234716c36a5e9fde15bae691c43a35
Reviewed-by: David Faure <david.faure@kdab.com>

1 files changed, 7 insertions, 4 deletions

diff --git a/src/widgets/styles/qpixmapstyle.cpp b/src/widgets/styles/qpixmapstyle.cpp
index e973a96a91..ce37065fb6 100644
--- a/src/widgets/styles/qpixmapstyle.cpp
+++ b/src/widgets/styles/qpixmapstyle.cpp
@@ -819,11 +819,14 @@ void QPixmapStyle::drawProgressBarFill(const QStyleOption *option,
         drawCachedPixmap(vertical ? PB_VComplete : PB_HComplete, option->rect, painter);
 
     } else {
-        if (pbar->progress == 0)
+        if (pbar->progress == pbar->minimum)
             return;
-        const int maximum = pbar->maximum;
-        const qreal ratio = qreal(vertical?option->rect.height():option->rect.width())/maximum;
-        const int progress = pbar->progress*ratio;
+        const auto totalSteps = qint64(pbar->maximum) - pbar->minimum;
+        const auto progressSteps = qint64(pbar->progress) - pbar->minimum;
+        const auto availablePixels = vertical ? option->rect.height() : option->rect.width();
+        const auto pixelsPerStep = double(availablePixels) / totalSteps;
+
+        const auto progress = static_cast<int>(progressSteps * pixelsPerStep); // width in pixels
 
         QRect optRect = option->rect;
         if (vertical) {