miniature view wrong on xfce workspace switcher applet

(Fedora 22 packaged xfce4-panel-4.12.0-3.fc22.x86_64)
I have two monitors, each has it's own panel, and each panel has it's own identically configured workspace switcher widget.
I have 9 workspaces, and the switcher widgets are configured to show them as 3 rows (so I have a 3x3 grid).

On one of my monitors, the workspace switcher always starts up correctly, on the other it always starts up showing a row of 9 workspaces. If I right click and bring up the properties dialog it immediately springs into the right 3x3 shape before I've done anything with the dialog.

Looking at .config/xfce4/xfconf/xfce-perchannel-xml/xfce4-panel.xml I see two
sections for pager data:

    <property name="plugin-4" type="string" value="pager">
      <property name="rows" type="uint" value="3"/>
    </property>

and

    <property name="plugin-7" type="string" value="pager">
      <property name="rows" type="uint" value="3"/>
    </property>

so they do agree.

(Actually; kind of side comment - why is the rows value associated with the panel widget? Given that there are shortcut binds for the window manager for up/down/right/left in workspaces it seems odd to have this setting on the panel widget).

Noticed in .xsession-errors:

libpager-Message: Setting the pager rows returned false. Maybe the setting is not applied.

I'm not sure when that was logged though.

that libpager message error seems to come from the call to

  wnck_pager_set_n_rows

and it can only return the error case if the wnck_pager_set_layout_hint fails.

I think I understand-ish why this is happening, and have a fix that makes it work, but I'm not sure if there are any side effects.
First the fix:

--- plugins/pager/pager.c 2015-02-26 20:32:17.000000000 +0000
+++ fixed/plugins/pager/pager.c 2015-05-31 16:23:35.682554450 +0100
@@ -320,8 +320,8 @@
   if (G_UNLIKELY (plugin->pager != NULL))
     {
       gtk_widget_destroy (GTK_WIDGET (plugin->pager));
- wnck_screen_force_update (plugin->wnck_screen);
     }
+ wnck_screen_force_update (plugin->wnck_screen);

   mode = xfce_panel_plugin_get_mode (XFCE_PANEL_PLUGIN (plugin));
   orientation =

---------------------

Now some explanation; I added a lot of debug in libwnck

wnck_pager_set_layout_hint: Entry for 0x26bb1e0
_wnck_pager_set_screen: entry for 0x26bb1e0
_wnck_pager_set_screen: Exit for !gtk_widget_has_screen
wnck_pager_set_layout_hint: pager->priv->screen==NULL
libpager-Message: Setting the pager rows returned false. Maybe the setting is not applied.
_wnck_pager_set_screen: entry for 0x26bb1e0
wnck_pager_set_layout_hint: Entry for 0x26bb1e0
wnck_screen_try_set_workspace_layout entry: screen=0x26938b0 current_token=0 rows=3 columns=0
wnck_pager_set_layout_hint: bottom 1
_wnck_pager_set_screen: Exit bottom
wnck_pager_set_layout_hint: Entry for 0x26bb620
_wnck_pager_set_screen: entry for 0x26bb620
_wnck_pager_set_screen: Exit for !gtk_widget_has_screen
wnck_pager_set_layout_hint: pager->priv->screen==NULL
libpager-Message: Setting the pager rows returned false. Maybe the setting is not applied.
_wnck_pager_set_screen: entry for 0x26bb620
wnck_pager_set_layout_hint: Entry for 0x26bb620
wnck_screen_try_set_workspace_layout entry: screen=0x26938b0 current_token=0 rows=3 columns=0
wnck_pager_set_layout_hint: bottom 0
_wnck_pager_set_screen: in !set_layout_hint branch, before get_workspace_layout n_rows=3
!!! _wnck_pager_set_screen: in !set_layout_hint branch, after get_workspace_layout n_rows=1
_wnck_pager_set_screen: Exit bottom
update_workspace_layout on 0x26938b0
!!! update_workspace_layout on 0x26938b0: num_workspaces=9 rows=3 cols=0
update_workspace_layout on 0x26938b0

Note the two lines with !!!
What I think happens is that the 1st pager instance works OK, but for the 2nd pager we end up in _wnck_pager_set_screen and it has:
  if (!wnck_pager_set_layout_hint (pager))
    {
      _WnckLayoutOrientation orientation;

      /* we couldn't set the layout on the screen. This means someone else owns
       * it. Let's at least show the correct layout. */
      _wnck_screen_get_workspace_layout (pager->priv->screen,
                                         &orientation,
                                         &pager->priv->n_rows,
                                         NULL, NULL);

that should ensure that the 2nd pager has the same layout, but it doesn't work because as from that debug we see:

_wnck_pager_set_screen: in !set_layout_hint branch, after get_workspace_layout n_rows=1

so _wnck_screen_get_workspace_layout is getting the wrong answer - why?
Well I think it's set by update_workspace_layout, and we only see that called
later in the debug; it's normally called on idle - see update_idle.
But I guess there's no idle between ...

hmm; hang off on that fix - it works if I kill the panel and restart it; however it still failed at start of session.

*** Bug 14643 has been marked as a duplicate of this bug. ***

*** Bug 15834 has been marked as a duplicate of this bug. ***

Indeed, I can confirm the bug. I'm not sure it's in the panel though, I think it's a limitation of libwnck.

From reading the code (and documentation) about that call only one pager widget should try to set the rows (i.e. you shouldn't really be able to configure two pager widgets differently consistently).
https://gitlab.gnome.org/GNOME/libwnck/blob/master/libwnck/pager.c#L2330

Also, I just noticed that Nick already understood this a looooong time ago. So marking this bug as a duplicate too...

*** This bug has been marked as a duplicate of bug 6555 ***