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

[Kevin Rushforth]

On Thu, 23 Sep 2021 19:01:52 GMT, Marius Hanl <mhanl at openjdk.org> wrote:

> This PR fixes an issue which is probably in JavaFX since VirtualFlow exists.
> While horizontal scrolling any VirtualFlow control the left blue border sometimes disappear/gets smaller. (see also image below)
> 
> This can be fixed by **snapping** the scroll bar value (similar like e.g. a **ScrollPane** does). 
> The same needs to be done on the table header as otherwise the column lines might be 1 px off to the cell lines. 
> As a side effect this also fixes that the column lines sometimes get's blurry when horizontal scrolling (see second image).
> 
> While testing with **-Dglass.win.uiScale** I found out that the problem is not fixed for a scale like 1.25 or 1.5, while it is fixed for 1 or 2. The border sometimes disappears only when the snapped value is a decimal number (which obviously does not happen on a scale of 1 or 2), e.g. something like 12.6 but it will never disappear when it's a normal number, so e.g. just 12.
> 
> That's why something like **Math.round(..)** or just a **cast** to an **int** instead of snapping fixes this problem for all scales. I also didn't notice any side effect. But not sure if this the right fix then. 
> How does JavaFX render a **node** when e.g. the x is a decimal number? And does a decimal number make sense (Why we e.g. don't round the value)?
> 
> Another explanation could also be that there is an issue somewhere deep inside the node layout/snapping/Clip/Group/pixel rendering and to simply round/cast the value just fixes the symptom here.
> 
> In any case I'm open for any feedback, help or explaination.
> I'm also glad for anything which might help identify the root cause here, if any.
> 
> ---
> 1.
> ![image](https://user-images.githubusercontent.com/66004280/134562508-bea6ab9d-d3d0-4dbb-b0ce-dc6570a94ed7.png)
> ---
> 2.
> ![image](https://user-images.githubusercontent.com/66004280/134563377-970b4e48-8528-4dad-95fb-fb93780413e8.png)

Given that this doesn't fix the problem when Hi-DPI scaling is in effect, this will need a different solution. See the discussion in [this thread](https://mail.openjdk.java.net/pipermail/openjfx-dev/2021-October/032215.html) on the openjfx-dev mailing list for some considerations regarding Hi-DPI scaling.

Since this PR isn't ready to be reviewed, I'm moving it to Draft.

modules/javafx.controls/src/main/java/javafx/scene/control/skin/VirtualFlow.java line 3082:

> 3080:             double snappedClipX = snapPositionX(clipX);
> 3081:             setLayoutX(-snappedClipX);
> 3082:             clipRect.setLayoutX(snappedClipX);

This is likely not the right place to snap the coordinates to a pixel boundary. This is usually done as part of layout. I'm also skeptical of the fact that you added it to `setClipX` but not `setClipY`.

-------------

Changes requested by kcr (Lead).

PR: https://git.openjdk.java.net/jfx/pull/630