RFR: JDK-8218745: TableView: visual glitch at borders on horizontal scrolling

[Marius Hanl]

On Wed, 10 Jan 2024 19:21:16 GMT, Marius Hanl <mhanl at openjdk.org> wrote:

> This PR fixes the border glitch/gap as explained in both linked tickets.
> 
> I noted that the `ScrollPane` control does not suffer from this problem, although the code is mostly the same as in `VirtualFlow`. The `ScrollPane` snaps the content correctly, no matter which scale. I carefully checked the code and it seems that the container structure in `ScrollPane` was explicitly written to handle this perfectly. There was definitely some thought on that.
> 
> So to finally fix this issue, I rewrote the `VirtualFlow` container/scrolling code to be **exactly** the same as in `ScrollPane`(Skin).
> And this also fixes the issue, while behaving the same as before.
> 
> In the future it may makes sense to try to somewhat unify the code from `ScrollPane` and `VirtualFlow`. I already have some ideas.
> 
> Unfortunately though, one more container is introduced to `VirtualFlow`, so the css needs to be changed since it is very strictly written in the first place:
> Before: `.list-view:focused > .virtual-flow > .clipped-container > .sheet > .list-cell`
> After: `.list-view:focused > .virtual-flow > .viewport > .clipped-container > .sheet > .list-cell`
> 
> To better understand this, the container structure (tree) is shown below:
> 
> - ScrollPane
>   - viewRect ->  `setClip` -> clipRect (viewContent size)
>     - viewContent -> `setLayoutX`
>       - Content
>   - vsb
>   - hsb
>   - corner
> 
> ---
> - VirtualFlow
>   - viewRect **(->NEW IN THIS PR<-)** -> `setClip` -> clipRect (clippedContainer size)
>     - clippedContainer/clipView -> `setLayoutX` (when scrolling)
>       - sheet
>         - Cell
>   - vsb
>   - hsb
>   - corner

Ok, so I found out the following: 
When a `Rectangle` is used as clip without any effect or opacity modification, the rendering goes another (probably faster) route with rendering the clip. That's why setting the opacity to `0.99` fixes the issue - another route will be used for the rendering.
This happens at the low level (`NGNode`) side of JavaFX. 

See here `NGNode`:
![image](https://github.com/openjdk/jfx/assets/66004280/6755574e-79e0-43a9-9830-93ea04715628)
`renderRectClip` seems to be doing something wrong.

I see that the clip rect is floored there, which seems to be the cause. Rounding it is fixing this for me.
The bug always appears when I scroll and the clip `RectBounds` are something like:
`RectBounds { minX:6.999996, minY:37.0, maxX:289.00003, maxY:194.0} (w:282.00003, h:157.0)`

while this does not happen when the value is something like:
`RectBounds { minX:7.000004, minY:37.0, maxX:289.0, maxY:194.0} (w:282.0, h:157.0)`

Note that the `minX` is floored down, that explains the 1px gap.

And now I fully understand why this only happens for the `VirtualFlow`:
This is the only container and location where the clip is layouted as well! (`setLayoutX`/`setLayoutY`).
In `ScrollPaneSkin`, there was the decision, that the clip is set and the content is layouted only, not the clip.

It may still make sense to me to actually imrpove the `VirtualFlow` here - it seems like a good choice to just set the clip and never touch it again and only change the underlying content layout, might be even a bit faster.

But there is obviously a bug on the low level `NGNode` side as well. Not sure where to go from here.

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PR Comment: https://git.openjdk.org/jfx/pull/1330#issuecomment-1987232133