In some cases the Move action on a Tabs Panels works, but the graphical interface shows that action as disabled.

Steps to reproduce:

• From user A:
  • Create two panels - editable by: User A
• From user B:
  • Create a "Tabs Panel" in a Dashboard
  • Attach the panels from User A (so, editable by, not you)

Then, from User B, see the Move actions that are grayed out:

But the Move actions actually works :) and that is OK.

I can reproduce this problem on this Dashboard where I attached some panels that are not mine, and the Move actions are disabled, but they work as expected:

https://phab.wmflabs.org/dashboard/view/12/

In short, the Tabs should be movable only if you can:

• Edit the parent Tabs Panel

But at the moment, these actions are disabled from the frontend also if you cannot edit the Sub-Panel themselves. But this is misleading, since you are not touching the Sub-Panel(s) at all, you are just showing many of them in a different order decided by you, that you created a parent Tabs Panel.

So the error is probably here:

$dropdown_menu->addAction(
  id(new PhabricatorActionView())
    ->setName(pht('Move Tab Left'))
    ->setIcon('fa-chevron-left')
    ->setHref($prev_uri)
    ->setWorkflow(true)
    ->setDisabled(($prev_key === null) || !$can_edit));

$dropdown_menu->addAction(
  id(new PhabricatorActionView())
    ->setName(pht('Move Tab Right'))
    ->setIcon('fa-chevron-right')
    ->setHref($next_uri)
    ->setWorkflow(true)
    ->setDisabled(($next_key === null) || !$can_edit));

So maybe the issue is just about removing the || !$can_edit, since that is an additional check not really intended, and also not consistent with the backend.

In short:

If you can Edit the Tabs Panel, of course you should also be able to Move things in it.