Middle mouse autoscroll on desktop

[MrRussLuther]

Summary

On desktop, holding the middle mouse button would start browser-style autoscroll: move the mouse away from where you pressed and the view scrolls that way, faster the further you move. Letting go stops it. It works on both axes at once, so on the series page you can scroll the episode row sideways and the page down in one motion.

Basic Example

With a mouse, getting through a long horizontal row means the arrow buttons or shift+scroll, one direction at a time, and long library grids mean a lot of wheel spinning. This would work like holding the middle button in Chrome or Firefox on Windows:

• Hold the middle button and move to scroll, release to stop
• A small marker shows where you started, with a dead zone around it so the view doesn't drift from small hand movements
• It scrolls the nearest horizontal and vertical scroll views under the cursor, so a row and the page it sits in both move

I'd hook it into the app's ScrollBehavior so it covers every scroll view without touching individual widgets. It doesn't take over any gestures, so left click, taps and "Drag using mouse" keep working as they do now. It'd be off on web, since the browser already has its own autoscroll there.

Drawbacks

It's always on for desktop, so the middle button over a scroll view is taken for autoscroll. Fladder doesn't use the middle button for anything today, so nothing is lost. A stray middle click just flashes the marker, since letting go stops it before anything moves. There's no click-to-toggle mode like browsers have, only hold. It also adds an overlay marker and a ticker while it's running.

Unresolved questions

Would you rather have this behind a setting next to "Drag using mouse" instead of always on?

Implementation

• I would like to implement this myself!

[MrRussLuther]

Opened #1169 with this implemented as described: hold only, origin marker with a dead zone, speed grows with distance, both axes, off on web. Always on for desktop for now; happy to put it behind a setting next to "Drag using mouse" if you'd prefer.