[circonian]

I can reproduce this 100% of the time without any addons & without doing anything special, just clicking on the label & selecting "/tell" does it every time.

It appears that in:

function SharedChatSystem:ShowTextEntryMenu()
   ...
   if data.target then
      AddMenuItem(switch, function() self:StartTextEntry(switch .. " ") end, nil, nil, itemColor)
   else
      AddMenuItem(switch, function() self:SetChannel(data.id) end, nil, nil, itemColor)
   end
  ...
end

The "/tell" switch is the only one that requires a target & thus the only one that gets added to the menu with:

AddMenuItem(switch, function() self:StartTextEntry(switch .. " ") end, nil, nil, itemColor)

But since they are not passing the channel to the function:

function SharedChatSystem:StartTextEntry(text, channel, target)
    if not self.currentChannel or channel then
        self:SetChannel(channel or CHAT_CHANNEL_SAY, target)   
    end
 ...
end

That code never runs & the channel never gets set.

The function:

function SharedChatSystem:SetChannel(newChannel, channelTarget)

is also the function that assigns focus to the control. Since the channel never gets set for the "/tell" switch, it never gets focus either.

It looks like it should have been:

function SharedChatSystem:ShowTextEntryMenu()
   ...
   if data.target then
      --==== it should have also passed the channel here (data.id) ====--
      AddMenuItem(switch, function() self:StartTextEntry(switch .. " ", data.id) end, nil, nil, itemColor)
   else
      AddMenuItem(switch, function() self:SetChannel(data.id) end, nil, nil, itemColor)
   end
  ...
end