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Bug
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Resolution: Unresolved
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P3: Somewhat important
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None
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4.7.0, 4.8.0, 5.0.0
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None
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Windows, Linux and Mac Os X
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macOS
My program uses big QSlider (from 700 pixels to 36000 pixels, maybe more. And I know widget max pixel size is something like 0XFFFFF0 pixels)
I observed a strange behavior with slider handle rendering with big QSlider.
Sometime slider handle just disappears after passing some position (using mouse or arrow keys), sometime slider handle "loop-back" to a pseudo-random position.
This problem is "only" a rendering graphics problem : QSlider internal value is still correct.
I investigated and found the reason.
In QStyle::sliderPositionFromValue(), implementation precision fluctuates depending on parameters values.
Depending on QSlider min, max and span values, QStyle::sliderPositionFromValue() returned value is limited in some different ways.
See my comments for the 3 cases of QStyle::sliderPositionFromValue() returned value computation :
int QStyle::sliderPositionFromValue(int min, int max, int logicalValue, int span, bool upsideDown)
{
if (span <= 0 || logicalValue < min || max <= min)
return 0;
if (logicalValue > max)
return upsideDown ? span : min;
uint range = max - min;
uint p = upsideDown ? max - logicalValue : logicalValue - min;
if (range > (uint)INT_MAX/4096) {
double dpos = (double(p))/(double(range)/span);
return int(dpos);
// case 1/ => computed with double precision :
// returned max value is INT_MAX
} else if (range > (uint)span) {
return (2 * p * span + range) / (2*range);
// case 2/ computed with uint precision :
// (2 * p * span + range) < UINT_MAX
// returned max value is (UINT_MAX - range) / (2 * range)
} else {
uint div = span / range;
uint mod = span % range;
return p * div + (2 * p * mod + range) / (2 * range);
// case 3/ => computed with int precision :
// (p * div + (2 * p * mod + range)) < INT_MAX
// returned max value is INT_MAX - (2 * range * mod + range) / range
}
// equiv. to (p * span) / range + 0.5
// no overflow because of this implicit assumption:
// span <= 4096
}
The bigger the range is, the bigger the return can be (case 1/)
I found this bug in qt 4.7 and qt 4.8. I think it is still there in qt 5.0, according to http://qt.gitorious.org