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

[Marius Hanl]

On Fri, 24 Sep 2021 12:42:31 GMT, Kevin Rushforth <kcr 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)
>
> 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`.

I didn't set **clipY** because it's not part of the story and **clipY** is always set to 0 when the `VirtualFlow` is vertical (which is the default for all `VirtualFlowContainer`. Only `ListView` **can** be non-vertical - This should be another bug and test case). That's why there is no issue with it normally.

I'm not sure if this is the wrong place. E.g. `ScrollPaneSkin` also listens on the scrollbar **valueProperty** and sets the **snapped** scrollbar value with **setLayoutX** - which is very similar then here. We set the **layoutX** here as well (for clip and the container).

Finally: While I understand it does not fix every scale, isn't it at least a good first step? (When we agree this is the correct location here)

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