Unity8 default mouse wheel increment seen by Mir clients is 7.0 (it should be 1.0)

Good news: Confirmed I can reproduce the bug.

Bad news: The bug only happens in Unity8 and not in Mir demo servers.

What changed:
Xmir 2:1.18.4-1ubuntu4 adds accurate vscroll/touchpad support. So instead of synthesising vscroll changes of any magnitude into fake button 4/5 press/releases, it now passes the vscroll float value straight down to the app.

This works well in Mir demo servers (with high-resolution touchpads too), but in Unity8 Xmir ends up sending about 7 button 4/5 events per mouse wheel tick.

It appears the flaw is in Qt/Unity8 code where a vscroll value of 1.0 (which means one mouse wheel tick) is not correctly maintaining the vscroll value of 1.0 after it passes through Unity8.

So the Xmir change is correct, but this is at least the third Unity8/Qt scrolling bug I've seen today. See also bug 1607223 and bug 1605513.

Status changed to 'Confirmed' because the bug affects multiple users.

Confirmed the wheel vscroll value received by Xmir is exactly 7.0:
       vscroll = mir_pointer_event_axis_value(pev, mir_pointer_axis_vscroll);

So for each mouse wheel tick:
USC sends a vscroll value of +1/-1 to Unity8
Unity8 sends a vscroll value of +7/-7 to apps

In previous versions of Xmir, we would ignore the magnitude and convert the 7 into 1. But now Xmir does proper smooth scrolling we need to honour the magnitude. So a value of 7 from Unity8 now results in 7x more scrolling.

The Xmir change is correct and we need to fix some Untiy8/Qt code somewhere so that a mouse wheel tick is always vscroll=1.0.

Test case:

env MIR_CLIENT_INPUT_RECEIVER_REPORT=log mir_demo_client_egltriangle -- --desktop_file_hint=unity8

And roll your mouse wheel over it.

Expected: vscroll values of +1/-1
Observed: vscroll values of +7/-7

Found the solution!

System Settings > Mouse & Touchpad >
Slide the second (unlabelled) slider all the way left, and then one notch right.

Turns out that unlabelled slider is the wheel scroll speed. Setting it one notch from the left correctly reduces the wheel speed from 7.0 to 1.0 as Mir clients expect.

It appears we have flaws (incorrect multiplication) in both QtMir and our default System Settings:

1. QtMir is multiplying vscroll events by 15, and forgetting to un-multiply them before passing them down to clients.

2. Our default System Settings (the unlabelled second slider under Mouse & Touchpad) seems to have a default scale of around 0.5. So the vscroll values of 15 from QtMir get converted to +7/-7 and that's what Mir apps receive in Unity8 instead of the expected +1/-1.

gsettings-ubuntu-touch-schemas has the default values

Status changed to 'Confirmed' because the bug affects multiple users.

This bug was fixed in the package gsettings-ubuntu-touch-schemas - 0.0.7+16.10.20160809-0ubuntu1

---------------
gsettings-ubuntu-touch-schemas (0.0.7+16.10.20160809-0ubuntu1) yakkety; urgency=medium

  [ Daniel van Vugt ]
  * Correct the default scroll speed multiplier which should be 1.0
    after (LP: #1607240)

 -- Michał Sawicz <email address hidden> Tue, 09 Aug 2016 23:00:26 +0000

This bug was fixed in the package qtmir - 0.4.8+16.10.20160810-0ubuntu1

---------------
qtmir (0.4.8+16.10.20160810-0ubuntu1) yakkety; urgency=medium

  [ Alan Griffiths ]
  * Reduce MirServer to an implementation detail of QMirServer (nothing
    else needs to touch it).

  [ Albert Astals Cid ]
  * Improvements from running clazy over the code
  * Compile with clang

  [ Daniel d'Andrada ]
  * Remove Application.stage and RoleStage from ApplicationManager

  [ Daniel van Vugt ]
  * Fix incorrect mouse wheel/touchpad scrolling scale (LP: #1607240)
    (LP: #1607223, #1607240)

 -- Michał Sawicz <email address hidden> Wed, 10 Aug 2016 06:53:16 +0000

This bug was fixed in the package qtubuntu - 0.63+16.10.20160809-0ubuntu1

---------------
qtubuntu (0.63+16.10.20160809-0ubuntu1) yakkety; urgency=medium

  [ Daniel van Vugt ]
  * Use correct scrolling scale of 120 as documented: (LP: #1607223,
    #1607240)

  [ Gerry Boland ]
  * Reapply rev 324 plus fix: EGL convenience, plus workaround for
    hybris not supporting GLESv3. (LP: #1507817, #1594198)
  * Logging: separate graphics & cursor logging into their own logging
    categories

 -- Michał Sawicz <email address hidden> Tue, 09 Aug 2016 23:01:21 +0000

This bug was fixed in the package ubuntu-system-settings - 0.4+17.04.20161109.1-0ubuntu1

---------------
ubuntu-system-settings (0.4+17.04.20161109.1-0ubuntu1) zesty; urgency=medium

  [ Daniel van Vugt ]
  * Allow scroll speed multipliers up to 3.0; since the default speed in
    (LP: #1607240, #1627567)

  [ Ken VanDine ]
  * Updated example plugin to work with current SDK
  * Fixed pep8 failure

  [ Timo Jyrinki ]
  * Stop depending on transitional packages. (LP: #1583079) (LP:
    #1583079)

  [ jonas-drange ]
  * Drop “at” from “Updated at <date>” string

 -- Ken VanDine <email address hidden> Wed, 09 Nov 2016 15:27:26 +0000