2040252 - Consider disabling scroll axis lock in WebDriver

Status: RESOLVED FIXED

(Remote Protocol :: Agent, enhancement, P2)

Description

[Hiroyuki Ikezoe (:hiro)]

This is a blocker to pass one of scroll interop 2026 wpt (bug 2015367).

Claude Code summarized:

Axis-locking is a UX accommodation for human input: when a finger or trackpad
gesture is near horizontal or near vertical, the engine zeros the
cross-axis delta so a slightly-tilted swipe doesn't drift diagonally. The exact
shape — what input source triggers it, what threshold engages the lock, when
the lock breaks — varies per engine.

Engine	Mechanism	Input source	Layer	Lock angle	Platforms
Chrome	SnapScrollController	raw touchscreen MotionEvent	gesture-detection (pre-gesture)	~38.7° (kMinSnapRatio = 1.25)	Android, Aura
touchscreen
Chrome	MouseWheelRailsFilterMac	trackpad wheel events with phase	wheel pipeline	~26.6° (kMinRailRatio = 2.0)	macOS only
WebKit	WheelEventDeltaFilterMac (_NSScrollingPredominantAxisFilter)	trackpad wheel events with Began/Changed phase	wheel pipeline	OS -defined (AppKit private)	macOS
WebKit	BasicWheelEventDeltaFilter	wheel events with Began/Changed phase	wheel pipeline	strict (3-event unanimity)	GTK, WPE, Win
Firefox	APZ axis lock	any PanGestureInput or touch	APZ	30° (apz.axis_lock.lock_angle = π/6)	all

Cross-engine punchline for WebDriver scrolls: Chrome and WebKit both
bypass their axis-locks naturally, because synthesized wheel events carry no
phase information and the lock is gated on phase. Firefox's APZ axis-lock has
no phase gate — it fires on any PanGestureInput — so a per-session pref
disable is required to match.

Comment 1

[Hiroyuki Ikezoe (:hiro)]

Attachment: Bug 2040252 - Disable axis lock in WebDriver. r?#webdriver-reviewers!,botond!

Comment 2

[Henrik Skupin [:whimboo][⌚️UTC+2]]

Hiro, I assume this only affects the touch events when dispatched asynchronously via the parent process, right? We probably should mark this bug as a blocker for bug 1773393 as well.

Comment 3

[Hiroyuki Ikezoe (:hiro)]

(In reply to Henrik Skupin [:whimboo][⌚️UTC+2] from comment #2)

Hiro, I assume this only affects the touch events when dispatched asynchronously via the parent process, right? We probably should mark this bug as a blocker for bug 1773393 as well.

This will also affect WebDriver's wheel actions via the parent process, since in bug 2015367 we are going to handle wheel actions as pan gestures just like what Chrome and Safari do.

Comment 4

[pstanciu]

https://hg.mozilla.org/mozilla-central/rev/e2a8c865ceb0