middle_click_autoscroll

Middle-click autoscroll for Flutter desktop & web. This package brings browser-style middle-mouse-button autoscroll to Flutter scrollables through one widget, MiddleClickAutoscroll. Press the mouse wheel, move the pointer away from the anchor and the content scrolls, faster the further you move. Click again (or release after dragging) to stop.

Flutter's scrollables do not do this on their own; this package fills the gap reported in flutter/flutter#66537. It is pure Dart: no platform channels, no native code.

The web example: middle-click drops the anchor, moving the pointer away scrolls (faster the further you go), moving to the other side scrolls back, and a click stops it.

• Wraps any subtree: ListView, GridView, CustomScrollView, SingleChildScrollView, PageView, TwoDimensionalScrollView, or drives a ScrollController you pass.
• Both axes at once, independently.
• Nested scrollables: the innermost scrollable under the pointer that can move in the requested direction wins; when it reaches its end, the next outer one takes over.
• Modes: auto (what Chrome does), toggle, hold.
• Dead zone (default 15 px, as in Chromium), pluggable speed curve (default modeled on Chromium: 0.011 · d^2.2 px/s), max speed.
• Frame-based scrolling with a Ticker and clamped jumpTo: never overscrolls, respects NeverScrollableScrollPhysics, lets physics settle when it stops.
• Stops on Escape, focus loss, pointer leaving the window, mouse wheel, any click (the stopping click is swallowed, like in browsers).
• Classic anchor indicator (circle with arrows) or your own via a builder.
• Directional mouse cursors (allScroll, resizeUp, resizeDownLeft, ...).
• Web: blocks the browser's own middle-click autoscroll and the Linux primary-selection paste inside the Flutter view (best effort).
• Only mouse input with exactly the middle button starts it; touch, stylus and trackpad are ignored.

dependencies:
  middle_click_autoscroll: ^0.1.1

import 'package:middle_click_autoscroll/middle_click_autoscroll.dart';

MiddleClickAutoscroll(
  child: ListView.builder(
    itemCount: 1000,
    itemBuilder: (context, i) => ListTile(title: Text('Item $i')),
  ),
);

Wrap high in the tree (for example around a whole page) and every scrollable below gets autoscroll; the one under the pointer is chosen at press time.

MiddleClickAutoscroll(
  mode: AutoscrollMode.toggle,          // auto | toggle | hold
  deadZoneRadius: 20,                   // px
  speedCurve: const LinearAutoscrollSpeedCurve(pixelsPerSecondPerPixel: 8),
  maxSpeed: 4000,                       // px/s per axis
  controller: myScrollController,       // optional: drive this instead
  indicatorBuilder: (context, details) => MyAnchor(details: details),
  cursorResolver: (details) => SystemMouseCursors.move,
  preventBrowserDefaults: true,         // web only
  onAutoscrollStart: (details) {},
  onAutoscrollEnd: () {},
  child: ...,
);

Speed curves: AutoscrollSpeedCurve.chrome, LinearAutoscrollSpeedCurve, PowerAutoscrollSpeedCurve, CallbackAutoscrollSpeedCurve((d) => ...) or your own subclass. The pure helpers computeAutoscrollVelocity and computeAutoscrollDirection are exported for custom indicators.

AutoscrollDetails (given to builders and callbacks) contains the anchor, pointer, current velocity, snapped AutoscrollDirection, which axes can scroll and whether the autoscroll is latched.

• The widget only sees scrollables below it in the tree.
• NestedScrollView's coordinated header/body scrolling is not reproduced: its inner and outer positions are scrolled independently.
• While the button is held, desktop OSes keep delivering drag events outside the window, so hold mode continues there (as in browsers). Latched autoscroll stops when the pointer leaves the window.
• Web: the browser has the final word. The package cancels middle-button mousedown/mouseup and any paste within 500 ms of a middle click inside the flutter-view, which stops Chrome's and Firefox's autoscroll and the Linux primary-selection paste in current browsers, but extensions, browser settings or future browser changes may still act. It cannot affect other apps or the X11/Wayland primary selection itself.
• Without an Overlay ancestor the indicator is drawn over the widget itself and a stopping click outside the widget is not swallowed.
• onAutoscrollEnd is not called when the widget is disposed mid-scroll.
• Indicator widgets must have a finite size.

example/ contains a desktop/web app with vertical, horizontal, 2D grid, nested and controller-driven tabs, and live controls for every option.