Edit: this post previously stated that the mouse buttons didn't work at all. I only recently discovered that touching the touchpad can make the left button work like a clickpad.
I just bought a Dell Precision 7550, which has 3 separate physical mouse buttons beneath the touchpad. It came with Ubuntu 18.04, where these mouse buttons work as intended. However, I just did a fresh install of 20.04 alongside it, and the mouse buttons have gone funny: I can't make the right button do anything, while the middle button works normally. The left button behaves like the click of a clickpad: if I'm not touching the touchpad, it does nothing. If I touch the touchpad at the same time, however, then it works: either it issues a left click or, if I'm touching the bottom right corner of the touchpad, it issues a right click.
How do I make Ubuntu stop treating this like a clickpad, and register the physical mouse buttons?
In the (working) 18.04, xinput shows
⎡ Virtual core pointer id=2 [master pointer (3)]
⎜ ↳ Virtual core XTEST pointer id=4 [slave pointer (2)]
⎜ ↳ HID 05ac:0256 Consumer Control id=12 [slave pointer (2)]
⎜ ↳ HID 05ac:0256 Mouse id=13 [slave pointer (2)]
⎜ ↳ DELL09C3:00 0488:120A Touchpad id=20 [slave pointer (2)]
⎜ ↳ PS/2 Generic Mouse id=25 [slave pointer (2)]
⎜ ↳ Microsoft Microsoft® 2.4GHz Transceiver v9.0 Consumer Control id=16 [slave pointer (2)]
⎜ ↳ Microsoft Microsoft® 2.4GHz Transceiver v9.0 Consumer Control id=19 [slave pointer (2)]
⎜ ↳ Microsoft Microsoft® 2.4GHz Transceiver v9.0 Mouse id=28 [slave pointer (2)]
⎣ Virtual core keyboard id=3 [master keyboard (2)]
↳ Virtual core XTEST keyboard id=5 [slave keyboard (3)]
↳ Video Bus id=6 [slave keyboard (3)]
↳ Video Bus id=7 [slave keyboard (3)]
↳ Power Button id=8 [slave keyboard (3)]
↳ Sleep Button id=9 [slave keyboard (3)]
↳ HID 05ac:0256 id=10 [slave keyboard (3)]
↳ HID 05ac:0256 System Control id=11 [slave keyboard (3)]
↳ Integrated_Webcam_HD: Integrate id=14 [slave keyboard (3)]
↳ Intel HID events id=21 [slave keyboard (3)]
↳ Intel HID 5 button array id=22 [slave keyboard (3)]
↳ Dell WMI hotkeys id=23 [slave keyboard (3)]
↳ AT Translated Set 2 keyboard id=24 [slave keyboard (3)]
↳ HID 05ac:0256 Consumer Control id=26 [slave keyboard (3)]
↳ Microsoft Microsoft® 2.4GHz Transceiver v9.0 System Control id=15 [slave keyboard (3)]
↳ Microsoft Microsoft® 2.4GHz Transceiver v9.0 Consumer Control id=17 [slave keyboard (3)]
↳ Microsoft Microsoft® 2.4GHz Transceiver v9.0 id=18 [slave keyboard (3)]
↳ Microsoft Microsoft® 2.4GHz Transceiver v9.0 Consumer Control id=27 [slave keyboard (3)]
Whereas the (broken) 20.04 shows:
⎡ Virtual core pointer id=2 [master pointer (3)]
⎜ ↳ Virtual core XTEST pointer id=4 [slave pointer (2)]
⎜ ↳ HID 05ac:0256 Mouse id=11 [slave pointer (2)]
⎜ ↳ HID 05ac:0256 Consumer Control id=13 [slave pointer (2)]
⎜ ↳ DELL09C3:00 0488:120A Mouse id=20 [slave pointer (2)]
⎜ ↳ DELL09C3:00 0488:120A Touchpad id=21 [slave pointer (2)]
⎜ ↳ PS/2 Generic Mouse id=27 [slave pointer (2)]
⎜ ↳ Microsoft Microsoft® 2.4GHz Transceiver v9.0 Mouse id=16 [slave pointer (2)]
⎜ ↳ Microsoft Microsoft® 2.4GHz Transceiver v9.0 Consumer Control id=18 [slave pointer (2)]
⎜ ↳ Microsoft Microsoft® 2.4GHz Transceiver v9.0 Consumer Control id=29 [slave pointer (2)]
⎣ Virtual core keyboard id=3 [master keyboard (2)]
↳ Virtual core XTEST keyboard id=5 [slave keyboard (3)]
↳ Video Bus id=6 [slave keyboard (3)]
↳ Video Bus id=7 [slave keyboard (3)]
↳ Power Button id=8 [slave keyboard (3)]
↳ Sleep Button id=9 [slave keyboard (3)]
↳ HID 05ac:0256 id=10 [slave keyboard (3)]
↳ HID 05ac:0256 System Control id=12 [slave keyboard (3)]
↳ Integrated_Webcam_HD: Integrate id=14 [slave keyboard (3)]
↳ DELL09C3:00 0488:120A UNKNOWN id=22 [slave keyboard (3)]
↳ Intel HID events id=23 [slave keyboard (3)]
↳ Intel HID 5 button array id=24 [slave keyboard (3)]
↳ Dell WMI hotkeys id=25 [slave keyboard (3)]
↳ AT Translated Set 2 keyboard id=26 [slave keyboard (3)]
↳ HID 05ac:0256 Consumer Control id=28 [slave keyboard (3)]
↳ Microsoft Microsoft® 2.4GHz Transceiver v9.0 id=15 [slave keyboard (3)]
↳ Microsoft Microsoft® 2.4GHz Transceiver v9.0 System Control id=17 [slave keyboard (3)]
↳ Microsoft Microsoft® 2.4GHz Transceiver v9.0 Consumer Control id=19 [slave keyboard (3)]
↳ Microsoft Microsoft® 2.4GHz Transceiver v9.0 Consumer Control id=30 [slave keyboard (3)]
The Microsoft stuff is my USB mouse/keyboard. The only meaningful difference I see is the appearance of DELL09C3:00 0488:120A UNKNOWN on 20.04, suggesting some hardware isn't recognized, although everything that's recognized on 18.04 is also recognized on 20.04. /etc/X11 is nearly identical, with only some changes in Xsession.d/70im-config_launch, Xsession.d/90qt-a11y, app-defaults/XLogo, and app-defaults/XCalc, none of which look relevant, except possibly the removal of QT_LINUX_ACCESSIBILITY_ALWAYS_ON in 90qt-a11y.
