RFR: 8309460: JScrollBar leaves behind clutter for non-integer UI scales

[Alexey Ivanov]

On Thu, 18 Jan 2024 11:09:12 GMT, Prasanta Sadhukhan <psadhukhan at openjdk.org> wrote:

> Lines  are left behind when moving the scrollbar in the positive direction. but are cleaned up on mouse release. 
> Additonally, with right arrow clicks too, the lines are left behind.
> Seems like for mouseDragged and scrollByUnit, the dirty region of the scrollbar is not repainted leading to artifacts
> which is being done in this fix..

> > Because float scale results in fractional pixels?
> > If the scale is 1.25, then 1 pixel is 1.25 physical pixels; 2 pixels are 2.5 physical pixels and so on.
> 
> Initially this issue looked similar to fractional scaling issues such as border rendering issues observed on windows previously but it seems to happen only on non-standard fractional UI scales such as 2.3, 2.7 and not on standard fractional scales supported on windows (1.25,1.75,2.25 ..) [#17484 (review)](https://github.com/openjdk/jdk/pull/17484#pullrequestreview-1834275178).

The underlying problem is still the same: there are fractional pixels.

I haven't run the test case myself. The attached screenshot has a pattern… not every pixel remains visible. Likely, these pixels are where the fractional pixels add up to produce a new full pixel.

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PR Comment: https://git.openjdk.org/jdk/pull/17484#issuecomment-1908921963