Smooth scrolling in SWT, idea and implementation

[Coderr123]

The current state of scrolling in SWT isn't great. ScrolledComposite only supports line scrolling, not pixel scrolling. Pixel scrolling can be implemented with MouseWheelEvent but it doesn't grab values with great enough precision for it to feel native enough using a trackpad. On macOS, SWT's mouse-wheel handling reads NSEvent.deltaX()/deltaY() — the legacy line scroll API, rather than the high-resolution scrollingDeltaX()/scrollingDeltaY() properties Apple introduced in 10.7 Lion, and doesn't look at hasPreciseScrollingDeltas(), phase(), or momentumPhase() at all.

Where this lives in the code

• bundles/org.eclipse.swt/Eclipse SWT/cocoa/org/eclipse/swt/widgets/Composite.java (and Control.java) — the scrollWheel(long id, long sel, long theEvent) callback, which currently reads nsEvent.deltaX()/deltaY() and multiplies by ScrollBar.getIncrement().
• bundles/org.eclipse.swt/Eclipse SWT PI/cocoa/org/eclipse/swt/internal/cocoa/NSEvent.java — the typed native wrapper, which does not currently bind scrollingDeltaX, scrollingDeltaY, hasPreciseScrollingDeltas, phase, or momentumPhase.

Related history:
SWT: Bugzilla [#220175](https://bugs.eclipse.org/bugs/show_bug.cgi?id=220175).

AWT: Similar discussions have taken place with regards to Java's AWT. This is one by a developer at JetBrains. mail.openjdk.org/pipermail/awt-dev/2017-January/012531.html

Other platforms

I know for a fact that Windows has similar values in their own API but I do not know the state of this on the Linux side. So if anyone knows more, feel free to add information to this discussion.

Proposed approach
My idea would be to expose these values and the momentum phase information along with similar values for Windows and Linux, depending on support. The question is however, would this be feasible and something that would genuinely be accepted in some form and if so, in what form? I haven't looked too much into this, so if I'm missing/misunderstand something, you can correct me.

[Coderr123]

After looking more into this and getting more familiar with the codebase, I realised that scrollingDeltaY(), although a nice value to have, is not really what can enable proper smooth scrolling. After implementing it in my own fork, I noticed that it has some weird issue where scrolling ends more abruptly than I would expect. Whether I'm misunderstanding the value or whether there is something in SWT causing this, I don't know. It seems however that this value isn't what Apple wants you to use to implement custom scrolling. To implement it there are two ways that can be explored. Nr. 1 is to create a way to bypass the custom line scrolling implementation in ScrollingComposite (or create a new SmoothScrollingComposite, likely more friendly for the other platforms), or it's to use this on the Cocoa side and then figure out ways to implement something similar in Win32 and GTK. Removing the custom scrolling implementation in ScrollingComposite and adding a method to properly set the NSScrollView size, automatically enables macOS's native scrolling, and it currently works well in the fork I have implemented this (very haphazardly though just for testing purposes).

Addition: Doh. I completely misunderstood this. What I referenced there in the AppKit documentation is for the "back" navigation gesture, when swiping left or right. So it seems that scrollingDeltaY is the desirable value, and I just need to fix whatever issue is causing it not to work as expected

I will continue exploring this for macOS and I think this seriously needs to be added to keep SWT modern.