SideMenu cannot be opened/closed using swipe gesture

[gentledepp]

Plan: Swipe-from-Left Gesture to Open/Close SideMenu

Context

Currently, the SideMenu (global, in ShellView) can only be opened via button taps in the
NavigationBar. There is no gesture-based opening or closing. The goal is to add Android/iOS/Windows-style swipe gesture support:

• Swipe-to-open: Drag from left edge to reveal the side menu or pane
• Swipe-to-close: Drag the open pane back to the left to dismiss it
• Input: Both touch and mouse (mouse useful for desktop testing)
• Platforms: Android, iOS, Windows (all platforms)

Both SideMenu use Avalonia's SplitView control under the hood, controlled via OpenPaneLength and IsPaneOpen properties.

[gentledepp]

Approach: Manipulate OpenPaneLength During Drag

During the swipe gesture, we set IsPaneOpen = true with OpenPaneLength = 0, then continuously update OpenPaneLength to match finger
position. Avalonia's SplitView does not animate programmatic OpenPaneLength changes — it simply re-layouts — so this produces smooth
finger-tracking. On release, we animate to the final state (fully open or closed).

New File

src/AvaloniaInside.Shell/SwipeOpenGestureHandler.cs

Reusable, delegate-based gesture handler class. No direct dependency on ShellView/Page internals.

Constructor parameters (all delegates/funcs):

• Control hitTestArea — the control to listen for pointer events on (the ShellView or Page itself)
• Func canSwipeOpen — returns whether swipe-to-open is allowed
• Func canSwipeClose — returns whether swipe-to-close is allowed
• Func isPaneOpen — returns current pane state
• Func targetWidth — returns the full pane width (SideMenuSize)
• Action setOpenPaneLength — sets OpenPaneLength during drag
• Action setPaneOpen — sets IsPaneOpen on the SplitView
• Action commitState — commits final open/closed state (sets SideMenuPresented / IsPaneOpen)

Gesture logic — two modes:

Swipe-to-Open (pane is closed)

1. Start zone: PointerPressed in leftmost ~30dp edge zone. Both touch and mouse accepted.
2. Direction lock: After ~10px movement, check if gesture is more horizontal than vertical. If vertical-dominant, cancel.
3. During drag: Set IsPaneOpen = true, clamp deltaX to [0, targetWidth], call setOpenPaneLength(clampedX). Record velocity samples.
4. On release: If velocity > ~800px/s rightward OR position > 40% of targetWidth → snap open. Otherwise snap closed.

Swipe-to-Close (pane is open)

1. Start zone: PointerPressed anywhere on the pane area or on the overlay/dimming region.
2. Direction lock: Same — must be more horizontal than vertical, and moving leftward.
3. During drag: currentWidth = targetWidth - |deltaX|, clamped to [0, targetWidth], call setOpenPaneLength(currentWidth).
4. On release: If velocity > ~800px/s leftward OR remaining width < 60% of targetWidth → snap closed. Otherwise snap back open.

Shared

• Snap animation: Animate OpenPaneLength from current to target over ~200ms with ease-out cubic.
• RTL support: If FlowDirection == RightToLeft, mirror edge zone to right side and invert direction logic.
• Input: Accept both PointerType.Touch and PointerType.Mouse.

Lifecycle:

• Attach() — subscribe to pointer events using AddHandler with RoutingStrategies.Tunnel (to intercept before content)
• Detach() / Dispose() — unsubscribe