I'm still having problems with this in 9.0.0.7. 
I use the command "Explore Contents" in Sandboxie and it opens a new Lister despite having "Open external folders in a new tab" checked.
Could it be an issue specific to Sandboxie? I don't know how Sandboxie launches Opus but if it's launching Opus in some kind of sandbox or as another user or whatever then it would explain it.
From their forums it seems that the program does launch things in a non-standard way:
sandboxie.com/phpbb/viewtopic.php?t=267
Perhaps it can be worked around in Opus but I expect GPSoft would need the people who make Sandboxie to explain exactly how Opus is being launched by their program.
Thanks for the prompt response Nudel.
Yes, I did wonder about that possibility given the functionality that Sandboxie is trying to provide, but Sandboxie does give you a visual indicator when it launches an app within a 'sandbox' (a [#] in the title pane of the window) and this doesn't appear to be the case when using "Explore Contents" (it also states that it has "no processes sandboxed at this time").
I guess we'll just have to wait and see whether posters earlier in this thread notice an improvement with 9.0.0.7.
If you want it fixed, ask the Sandboxie people to provide details of how they launch the folder. I think that's the only way we'll get to the bottom of it.
Good thinking Nudel. I've logged a post on the Sandboxie forums.
The Sandboxie devs have responded. The "Explore Contents" command does this:
ShellExecute(hwndofSandboxieControl, L"open", PathToSandboxFolder, NULL, PathToSandboxFolder, SW_SHOWNORMAL);
Does that shed any light on why we get a new lister instead of a new tab?
The L"open" should be NULL (ie, they should not be specifying an action, but instead should be asking for the default action)
Yeppers... and Jon should know 
if for no other reason since this type of behavior was based on a similar 'assumption' back in old versions of Opus v8.x which caused certain inconsistencies with how some media files (like mp3's) were treated on doubleclick between Explorer and Opus. This was because Opus similarly and explicitly 'opened' the files instead of calling for the 'default' shell action which for some types of files was NOT 'open' after being modified by other apps (like Winamp) to be something like 'Play' instead of 'open'...
That should be all there is to it... so time for Sandboxie devs to see if they're willing to make a change... I'm sure you can refer them to this topic if they want a little more info.
Thanks guys. This info has been passed on to the Sandboxie devs.
Now I'll just wait with my fingers crossed...
If you want something to back up the suggestion of using NULL instead of "open" then there's this by one of the Windows developers:
As of v2.0.0.7, I'm still having a problem here with a program spawning new listers instead of tabs.
Should I elaborate on that?
Or couldn't Dopus just handle the issue in a more generalized fashion? "If there's an open lister, open the folder in a new tab, regardless of what the external launch command was"
[quote="kingding"]As of v2.0.0.7, I'm still having a problem here with a program spawning new listers instead of tabs.
Should I elaborate on that?[/quote]
You will need to talk to the people who write the programs which are causing new windows to open and ask them to explain how they are launching the folders, then get back to us.
[quote="nudel"]You will need to talk to the people... [/quote]Um, I'll see what I can find out.
[quote="nudel"]... how they are launching the folders[/quote]But, I mean, as long as the program doesn't magically call "dopusrt.exe /cmd GO NEW ...", Dopus could handle the "open folder!" request in its own way, couldn't it?
Roboform is another program that opens new listers.
After downloading and installing it, click on the task symbol and choose "Profiles" -> "Explore Folder"
Maybe there's more we can do to improve this - I'll look into it.
I "thought" one issue was that on Vista, modifying the 'open' verb as defined in the registry in order to launch opus simply didn't work and it seemed that something in Windows was bypassing this method of controlling shellexecute... (or maybe shellexecute is intercepted and handled differently in vista as part of the native security enhancements)?
Looked at the other way... for XP users, can't they modify the 'open' verb in the registry to use Opus instead of explorer?
Yes, you can turn on the 'global_explorer_replacement' option in Prefs / Misc / Advanced - might be worth a try.
I'm using Win XP with 'Replace Explorer for all folders' switched on, if that helps.
Ok I have made some changes which will hopefully solve this problem for good. Certainly it now works with the specific issue mentioned above (Roboform). Next version should be out later this week.
Some more "opens new lister" examples:
Emule: "Shared Files" (main toolbar) -> Right click on file -> "Open folder"
Free Download Manager:Right click on file -> "Open folder"
Mediamonkey: Right click on file -> "Find more from same" -> "Folder (Explorer)"