Add a saved preference, invert_middle_drag (off by default), that
reverses the direction of the middle-button pan and of the latched
auto-scroll. It changes only the direction, not which of the two a
platform uses, so it composes with the Windows auto-scroll work.
Reachable as Preferences > Invert middle-button drag and headlessly
through ui.set invert_middle_drag. Parity "Automatic scrolling" notes
updated; status unchanged.
Tests: a unit test of both paths with the switch on and off (dead zone
kept), a settings round-trip test, and an end-to-end control-channel
drag. Each fails with its fix reverted.
Claude-Session: https://claude.ai/code/session_01SqmHYx8FiWM5PcsoiErgp9
Co-authored-by: KenM76 <KenM76>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>