[launcher] icons no longer active after an incomplete drag

@Jani: this looks like a pretty bad issue. Do you have a way to reliably reproduce it?

Olivier, indeed seems a bad issue, but I only saw it once. I think that I clicked the icon a bit like beginners do when learning mouse clicking - and dragged it a bit too. But I can't make it happen again.

This happened to me too: I dragged the Hamster icon, and it appeared to get stuck or frozen in mid-drag, partially overlapping the icon above it. The launcher icons become unresponsive after this. (The top bar and BFB (Ubuntu button) still responded normally.)

I have not been able to reproduce it yet, but will report back if i can.

Other notes:

- unity-2D version: 3.8.4.1-0ubuntu1
- Ubuntu version: 11.04, upgraded from 10.10.
- Graphics setup: GeForce FX 5500, with the Nouveau driver.
- This bug happened shortly after starting a Unity 2D session for the first time on this installation: i'm not sure if this affects the reproducibility of it. (I restarted since.)

It happened again. The launcher is not completely locked up, just can't launch apps.
Every app icon when clicked shows the app name tooltip. Right click has no effect.
This time it is the terminal icon overlapping the one under it.
Clicking on the blank black space left by the icon moving down has no effect.

backtrace info for threads while the app is in this state

It happened again here. I can confirm what Jani's saying: while launcher seems to be wedged, it still responds to desktop events like alerts/wiggles and applications being launched and closed. Clicking an icon triggers displaying the name tooltip (which is no longer triggered by hovering), but nothing else.

Possible clue: This time, i was alert enough to observe that when the bug was triggered, it happened while my system was busy swapping heavily enough for mouse cursor movement to become jerky. I suspect that the accidental "drag" may have been a normal click + mouse movement whose sequence of events got delayed or interfered with by the disk I/O. I'm not familiar with the code, but would an unusual or unexpected sequence of mouse / click events be able to put the launcher into this bad state?

Another note: I was using the NVIDIA driver this time, not the Nouveau one, so the problem doesn't seem to be related to them.

Bingo: I can reliably reproduce the bug now.

It involves quickly clicking an icon and moving the mouse away, nearly at the same time: if the mouse movement is too late, a normal click is performed, and if the click release is too late, the gesture turns into the normal drag / swipe gesture of the entire icon row.

It seems that thrashing the disk makes it easier to hit this edge case, but now that i know how, i can easily trigger it while the system is idle, too.

Can a developer advise me on how to generate debug logs or traces relating to icon mouse events?

Thanks for the useful comments Piet. I found a very reliable way to reproduce the bug:

1. Patch launcher/ListViewDragAndDrop.qml to change line 54 from:

        interval: 500 /* in milliseconds */

to:

        interval: 5000 /* in milliseconds */

2. Restart the launcher
3. Position the mouse pointer at the very bottom of one application item of the launcher
4. Press the left mouse button and keep it pressed
5. Move the mouse around 6-7 pixels down (not more so that the list does not start scrolling)
6. Release the left mouse button

Steps 5. and 6. should be performed in under 5 seconds.

Piet: a patch that should fix the issue is under review at https://code.launchpad.net/~fboucault/unity-2d/launcher_interrupted_drag/+merge/62987

I can reliably reproduce the issue following the steps described by Florian in comment #8.

Great. :)

I applied the patch, and tried for a minute or two to reproduce the problem as described in comment #7, but could not. After unapplying the patch, i could immediately reproduce it again. As far as i can tell, that fixes it.

Thanks, Florian!

This bug was fixed in the package unity-2d - 3.8.8-0ubuntu1

---------------
unity-2d (3.8.8-0ubuntu1) oneiric; urgency=low

  [ Didier Roche ]
  * new upstream release:
    - [spread] Corruption when switching workspaces after windows have been
      moved to other workspaces (LP: #760787)
    - [launcher] launcher won't fully paint, corrupted view (LP: #764690)
    - [launcher] icons no longer active after an incomplete drag (LP: #768812)
    - drag from dash to launcher (LP: #662616)
    - Don't create windows over the launcher (LP: #688816)
    - [launcher] Does not reveal when hovering over the left edge of the
      screen (LP: #760537)
    - UIFe: Launcher - update launcher reveal interaction to make it more
      accessible to first time users (LP: #754583)
    - [launcher] Trash icon should indicate when it has deleted elements
      (LP: #715611)
    - When windows open for the first time they should not hide the launcher
      (LP: #723878)
    - double clicks should be disabled on bfb/Place launcher icon/double key
      press (LP: #766776)
    - [dash][launcher] Should use real transparency when a compositing manager
      is running (LP: #794042)
    - [dash] Thunderbird icon is pixelated (LP: #767115)
    - [panel] Hovering the mouse cursor over the BFB reveals the current
      window’s menu (LP: #793403)
    - [panel] Hovering the mouse cursor away from the appmenu applet doesn’t
      hide the menu (LP: #793406)
    - unity-2d: does not parse QT_GRAPHICSSYSTEM env var (LP: #791852)
    - Cannot drag applications from dash to desktop (LP: #756614)
  * debian/control:
    - unity-2d-panel recommends the indicator, not unity-2d
    - appmenu-gtk and appmenu-qt are already provided by indicator-appmenu
    - remove other meaningless recommends
  * debian/libunity-2d-private0.install:
    - install everything in Unity2d private directory

  [ Florian Boucault ]
  * debian/unity-2d-launcher.install:
    - do not install usr/lib/qt4/imports/UnityApplications/ anylonger as all of
      UnityApplications features have been moved to the Unity2d QML plugin
      installed by libunity-2d-private0
  * debian/control:
    - unity-2d-places and unity-2d-spread do not depend on unity-2d-launcher.
      All they need is now in libunity-2d-private0.
 -- Didier Roche <email address hidden> Tue, 14 Jun 2011 16:14:18 +0200