Writing modes and physical axes

Tracker orientation represents a physical screen axis. It is independent from the logical flow created by CSS writing-mode and direction.

Native scroll offsets can use different signs and origins depending on writing mode, direction, axis, and browser behavior. Tracker normalizes those native coordinates into its physical X/Y track model.

Physical Tracker geometry vs logical CSS flow

Tracker orientation
Setting	Physical meaning	Rail edges
`vertical`	Screen Y axis	left / right
`horizontal`	Screen X axis	top / bottom

CSS and Tracker responsibilities
Property	Controls
`writing-mode`	logical flow
`direction`	logical direction
Tracker `orientation`	represented physical axis

Tracker physical axis Vertical — physical Y Horizontal — physical X

All four comparison Trackers use physical Y with a right rail.

Representative writing modes

These four profiles are representative, not an exhaustive matrix. The baseline and RTL pair isolate direction within horizontal writing, while the two vertical profiles exercise substantially different native physical scrolling behavior.

Drag any rail toward its opposite physical edges to see the same physical Tracker model control each native scroller.

Runtime CSS flow changes

This isolated fifth Tracker deliberately disables automatic update sources. The host application changes CSS flow and then explicitly calls tracker.render() to synchronize physical marker geometry.

Apply a profile to the fifth Tracker

Initial profile: horizontal-tb / ltr. Host invalidation is owned explicitly.

writing-mode: horizontal-tb
direction: ltr
Tracker axis: physical Y
Rail: right

Target 1

Target 2

Target 3