scroll: add horizontal axis and orthogonal nested routing

scroll: add horizontal axis and orthogonal nested routing

Problem

Canvas scroll views currently support only vertical scrolling. The platform input contract already carries both delta_x and delta_y in types.zig, but the canvas event path in canvas_widget_events.zig forwards only the vertical component. The AppKit scroll handler is likewise implemented around a vertical offset in appkit_host.m.

As a result, a native trackpad gesture cannot navigate a horizontal timeline, calendar, or carousel. This becomes especially noticeable when a horizontally scrollable region contains a vertically scrollable child: the vertical child can scroll, but horizontal momentum over that child has no horizontal ancestor to reach.

Minimal reproduction

1. Create a scroll view whose content is wider than its viewport and configure it for horizontal scrolling.
2. Place a vertically scrollable region inside the horizontal region.
3. Hover the nested vertical region and perform a horizontal two-finger trackpad gesture.
4. Observe that the horizontal region does not move because the canvas event path discards delta_x.
5. Perform a vertical gesture and observe that the inner vertical region scrolls normally.

Expected behavior

• A horizontal gesture scrolls the deepest eligible horizontal region.
• A vertical gesture scrolls the deepest eligible vertical region.
• For an orthogonally nested pair, each gesture reaches the region matching its axis.
• Once a gesture chooses an axis, momentum remains locked to that axis so minor diagonal noise does not switch targets mid-gesture.
• When a same-axis inner region reaches its extent, the existing saturation handoff behavior continues to pass remaining movement to an eligible ancestor.

Proposed scope

Add a single-axis scroll contract:

• Introduce WidgetScrollAxis { vertical, horizontal }.
• Expose the axis through Native markup, for example axis="horizontal", and through Zig as ElementOptions.scroll_axis.
• Retain the existing scalar scroll offset and interpret it along the selected axis.
• Make scroll layout, content extents, semantics, keyboard behavior, scrollbar placement, and native driver updates axis-aware. The current layout path calculates vertical scroll geometry in widget_layout.zig, while the driver derives a vertical extent in canvas_widget_scroll_drivers.zig; both would select their active dimension from the new axis.
• Route each gesture to the deepest scroll region whose configured axis matches the gesture's locked dominant axis.
• Choose that axis from the first meaningful input sample and retain it through the gesture's momentum phase.
• Preserve the existing same-axis nested boundary handoff covered by the vertical nested-scroll tests in canvas_widget_scroll_tests.zig.

This keeps the change narrow: one scrollable axis per region, one scalar offset, and one axis-aware routing rule. Supporting simultaneous two-dimensional scrolling is not required.

Non-goals

The following application-level features should remain separate from this change:

• synchronized scroll groups
• content recycling or reach-boundary callbacks
• scrollbar visibility controls
• nested template iteration

Existing issues and pull requests

No existing issue or pull request found against upstream main at eefa369 covers general horizontal canvas scrolling or orthogonal nested routing. PR #90 and PR #83 concern text-field scrolling and do not appear to be the right threads to join.

Start with the scroll input path in src/runtime/canvas_widget_events.zig and the platform contract in src/platform/types.zig, then inspect axis-dependent layout and driver behavior in src/primitives/canvas/widget_layout.zig and src/runtime/canvas_widget_scroll_drivers.zig. Use the nested-scroll cases in src/runtime/canvas_widget_scroll_tests.zig and the AppKit handler in src/platform/macos/appkit_host.m as reference points. Done means horizontal and vertical regions route matching gestures, preserve locked-axis momentum and boundary handoff, and cover the axis-aware behavior.