feat: translate Shift+wheel into horizontal scroll on macOS/Windows/Linux

[FunJim]

Summary

Hold Shift while turning the mouse wheel to scroll horizontally in every app — on macOS, Windows, and Linux. Each platform implements the translation in its own mouse hook so the behavior is consistent across the trio.

Why

The OS doesn't translate Shift+wheel to horizontal scroll system-wide — only some apps (mostly browsers) do it themselves. With Mouser active, the user-expected "Shift+wheel = horizontal scroll" behavior is now consistent across every app.

Behavior

• Only triggers on a real vertical wheel event with Shift held — a hardware horizontal tilt-wheel still goes through the existing HSCROLL_LEFT/RIGHT dispatch path untouched.
• Trackpad / continuous scroll events are not affected.
• Direction is controlled by the existing Invert horizontal scroll setting (invert_hscroll), so no new UI is required.

Per-platform implementation

macOS (core/mouse_hook_macos.py)

• New _SHIFT_WHEEL_HSCROLL_MARKER so the synthetic horizontal scroll we post passes through our own CGEventTap on re-entry without being reprocessed.
• New _post_shift_hscroll_event helper copies axis-1 (vertical) deltas onto axis-2 (horizontal), zeros axis-1, copies scroll/momentum phases, and strips the Shift modifier from the new event's flags so apps that already do their own Shift+scroll translation (browsers, NSScrollView) don't double-translate.

Windows (core/mouse_hook_windows.py)

• New WM_APP_INJECT_SHIFT_HSCROLL deferred-injection message, mirroring the existing WM_APP_INJECT_HSCROLL invert path.
• GetAsyncKeyState(VK_SHIFT) is queried in the LL hook for every WM_MOUSEWHEEL; if Shift is held the original is blocked and a MOUSEEVENTF_HWHEEL is queued for the raw-input window thread to inject via SendInput.

Linux (core/mouse_hook_linux.py)

• Opens keyboard evdev devices (read-only, no grab) on first use to query active_keys() for KEY_LEFTSHIFT / KEY_RIGHTSHIFT.
• In _handle_rel, when Shift is held during REL_WHEEL, writes a translated REL_HWHEEL on the virtual mouse and swallows the original vertical scroll; matching REL_WHEEL_HI_RES events are also suppressed so vertical scroll doesn't double-fire.
• The virtual uinput device always exposes REL_HWHEEL so the translation works even on basic mice without a tilt wheel.

Test plan

• python -m py_compile main_qml.py core/*.py ui/*.py — passes
• python -m unittest discover -s tests -p "test_*.py" — 510 passed, 1 skipped (existing skip)
• New MacOSShiftWheelHScrollTests (7 cases) — translation, Shift stripping, marker passthrough, no-Shift / tilt-wheel no-ops, invert_hscroll both directions.
• New LinuxShiftWheelHScrollTests (7 cases) — keyboard discovery, _shift_held state, REL_WHEEL→REL_HWHEEL translation, passthrough without Shift, invert_hscroll direction, hi-res suppression, virtual-uinput REL_HWHEEL injection.
• Manual macOS verification by the author: Shift+wheel scrolls horizontally system-wide; toggling Invert horizontal scroll flips the direction.
• Windows / Linux manual verification — the Windows hook lacks a unit-test harness in this repo (existing pattern), so the change is best confirmed on hardware before merging.

Notes

• No config schema change.
• Keyboard discovery on Linux is one-shot and best-effort; if a keyboard is hot-plugged after start, it won't participate in Shift detection until the hook restarts. Acceptable for a first pass.