Your system information

• Steam client version (build number or date): Mar 22 2017
• Distribution (e.g. Ubuntu): Arch Linux
• Opted into Steam client beta?: No
• Have you checked for system updates?: Yes

Please describe your issue in as much detail as possible:

The buttons for mouse back (labeled 4 in Big Picture) and mouse forward (labeled 5) in the desktop Configuration for the Steam controller sends the wrong button events. It sends button 6 for back and button 7 for forward, while normal mouses sends 8 for back and 9 for forward. This can be checked with xev. This is general for all Linux distributions, not specific for Arch Linux. See e.g. here for Ubuntu, which lists:

# b:1  -  left mouse button
# b:2  -  wheel button
# b:3  -  right mouse button
# b:4  -  mouse wheel up
# b:5  -  mouse wheel down
# b:6  -  mouse wheel left
# b:7  -  mouse wheel right
# b:8  -  back side button
# b:9  -  forward side button

Steps for reproducing this issue:

1. Map mouse back and mouse forward to buttons on the Steam Controller for the desktop configuration (e.g. the default desktop configuration).
2. Go to a web page in a separate browser (not the browser inside Steam).
3. Press one of the mapped buttons.

Expected result:
The web browser should go back or forward one page.

Actual result:
The web browser scrolls left or right.