Add global “Scroll Speed” setting to Mouse & Touchpad

[matthias47493]

Summary
This PR introduces a new “Scroll speed” section in the Mouse & Touchpad settings panel.
It allows users to globally adjust the mouse wheel scrolling speed (similar to Windows’ “Number of lines to scroll each time” feature).

Motivation
Many users have requested finer control over the scroll wheel speed, especially when using high-resolution or fast-scrolling mice.
This feature makes Cinnamon more user-friendly and aligns it with other desktop environments and operating systems.

Implementation details

• Adds new GSettings key:
  org.cinnamon.settings-daemon.peripherals mouse-scroll-speed (integer, 1–10)

• Adds UI section: Scroll speed (horizontal GtkScale, 1–10)

• Adds Python controller (mouse_scrollspeed.py) that:

  • Reads/writes the GSettings key
  • Writes ~/.imwheelrc with adjusted scroll multipliers
  • Restarts imwheel automatically to apply the new scroll speed

• The implementation uses imwheel for Xorg environments as a reliable global workaround. A native Wayland implementation can be added later using libinput APIs.

File overview
data/org.cinnamon.settings-daemon.peripherals.gschema.xml
ui/mouse_scrollspeed_section.ui
src/mouse_scrollspeed.py
README.md

Notes for maintainers

• Tested on Linux Mint 22 Cinnamon, Xorg session.
• Requires package: imwheel
• On systems without imwheel, a dialog suggests installation via sudo apt install imwheel.
• The mapping (1–10 → scroll multiplier) is linear and can be adjusted easily in _map_value().

Future work

• Replace imwheel with a Cinnamon-native handler for libinput (Wayland compatibility)
• Add per-device scroll speed overrides

cinnamon-scrollspeed-extension.zip

[billyswong]

The problem is, imwheel is a hack. It intercepts mouse wheel events and emulate more frequent or less frequent event signal in place of "real" mouse wheel clicks.

Such hack is not reliable and not suitable for built-in setup. For example, in Google Map, I expect my mouse wheel scroll will zoom in or zoom out the map one step each click. Imwheel will break that.

[billyswong]

An ideal non-hackish solution:
GTK and/or Qt publish a way to get/set scroll wheel speed multiplier. Then other applications outside these 2 GUI toolkits read the config and emulate the same behaviour.

[SamStephens]

I'm impacted, my touchpad scroll is far too fast. We can see from the forums people are currently hacking their way around this lack of configurability: https://forums.linuxmint.com/viewtopic.php?t=411987 and https://forums.linuxmint.com/viewtopic.php?p=2399264. Could this pull request be updated to use xinput in the way these forum posts suggest? I'm brand new to Linux Mint and to using Linux with a GUI, so a lot of this is currently way over my head.