Touch interactions: unified spec and phased implementation (capability detection, touch resize, gesture dispatcher, left rail)

[btli]

Problem

Touch input across the web app and the Android/iOS shells is fragmented and partially broken:

• No pointer-capability detection anywhere — 'mobile' is inferred from viewport width alone, re-implemented with drifting thresholds (767 / 767.98 / 768 across at least 5 sites incl. the Android bridge). Touch laptops and tablets ≥768px are treated as pure mouse desktops.
• All five rail/panel resize hooks are mouse-only (useResizablePanel, useResizableSidebar, useResizableInlinePanel, useResizableCommentsPanel, useResizableColumn) — rails cannot be resized by touch at all.
• Gesture ownership is fragmented: dnd-kit's 250ms hold, Radix's ~700ms long-press, row swipe, edge swipe, and native back all compete for the same pointer stream with ad-hoc per-surface code. This is why the session-row touch PR train (feat(web): swipe session rows to archive or delete on touch #3154 → feat(web): swipe session rows to archive or delete on touch #3985, Keep mobile ungroup drop zone below sessions #4057/feat(web): project folder context menu on right-click and long-press #4060/fix(web): suppress session menu during touch drag #4065) had to be abandoned and rethought.

Approach

A holistic spec + design doc (PR #4736) defines the end-state: pointer-capability detection (TR-1..4), pointer-event resize with real touch targets (TR-5..9), a unified gesture dispatcher generalizing the train's useRowGesture (TR-10..21), and a phase-gated left-rail navigation model (TR-22..27), with accessibility and perf gates (TR-28..30).

Delivery (each PR references this issue)

• Spec + design doc: docs: holistic touch interaction spec and design (gesture dispatcher + left rail) #4736
• Phase 0 — capability detection + breakpoint consolidation: feat(web): single input-capability hook + canonical md breakpoint (touch TR-1..4) #4785
• Phase 0 — touch resize, five hooks: Migrate panel resize to pointer events #4784 (panel), feat(web): support pointer sidebar resizing #4780 (sidebar), fix(web): make the inline rail resizable via touch and stylus #4781 (inline), fix(web): migrate comments-panel resize to pointer events for touch #4786 (comments), fix(web): migrate useResizableColumn to pointer events with keyboard resize (touch P0) #4782 (column)
• Phase 0.5 — MD_BREAKPOINT rewire + hover-reveal touch fallback (TR-5): PRs to follow
• Phase 1 — gesture dispatcher + session-row consumers (supersedes feat(web): swipe session rows to archive or delete on touch #3985): PRs to follow
• Phase 2 — left rail (own go/no-go per the design doc): PRs to follow

A consolidated touch-interaction demo (Android emulator recording) will be attached to the implementation PRs at the validation stage.

[github-actions]

🤖 Automated triage

• Bot assessment: Medium impact
• Priority: P2-medium
• Why: This improves touch capability detection, resizing, gestures, and rail navigation for web users and iOS/Android shell users, but existing workflows remain generally completable through current navigation and mouse/desktop alternatives.

This automated assessment uses the issue content and repository signals. Maintainers can override the priority label.