Your system information

• Steam client version (build number or date): Jun 12 2018
• Distribution (e.g. Ubuntu): Manjaro Linux (64-bit)
• Opted into Steam client beta?: Yes
• Have you checked for system updates?: Yes

Please describe your issue in as much detail as possible:

I'm unable to assign Mouse 4 or Mouse 5 as the push-to-talk button in the Friends List Settings.

Unlike buttons like Shift, Alt, Tab and other, which aren't possible to assign ("None" is shown when I try to assign them), Mouse 4 and 5 buttons doesn't seem to be registered at all - the blue frame keeps flashing and text "Press key or mouse-button to assign" is still shown.

Also, a lot of buttons have a problem of assigning another button instead of itself. And the button shown in the settings (not the one I tried to assign) will be the one that actually is working as PTT.
Here's the list of buttons and what they are turning into in the settings:

Num 0 - `
Num 1 - A
Num 2 - B
Num 3 - C
Num 4 - D
Num 5 - E
Num 6 - F
Num 7 - G
Num 8 - H
Num 9 - I
Num * - J
Num + - K
Num - - M
Num . - N
Num / - O
F1 - P
F2 - Q
F3 - R
F4 - S
F5 - T
F6 - U
F7 - V
F8 - W
F9 - X
F10 - Y
F11 - Z
F12 - [
Left Super (Win) - [
Right Super (Win) - \
INS - -
DEL - .
PG DOWN - '
PG UP - 1
END - 3
HOME - 4
Left Arrow - 5
Up Arrow - 7
Down Arrow - 9
Right Arrow - '

Steps for reproducing this issue:

1. Go to the "Friend List Settings",
2. Go to the "Voice" tab,
3. In the "PUSH-TO-TALK" section, enable push-to-talk if it wasn't already,
4. Click on the "Push-to-talk hotkey assigned as" input field,
5. Press F1 (or any key from the list above)
6. Instead of F1 hotkey will be assigned to the P