Certain programs when launched (Microsoft Outlook for one) will cause the NextSTART taskbar to show and it won't go away. The task that alerted NextSTART will be in blue. Here's an example:

I launch Microsoft Outlook from Quick Start in NextSTART, although it doesn't matter where it's launched from. NextSTART hides and the small Outlook box appears while it loads my profile, etc. This box remains on the screen for about five seconds. Then the box disappears and the main Outlook window appears. At the same time, NextSTART appears with the Outlook task showing in blue. Clicking in the Outlook window doesn't make NextSTART hide. Closing Outlook or NextSTART will. But I have to open Outlook before restarting NextSTART or else NextSTART will appear and not go away again.

I've looked in every NextSTART setting but I can't find any way to stop this behavior.

Applications have a mechanism that allows them to 'flash' the button in the Windows taskbar when they require attention.

Winstep applications "listen" to this same mechanism and will flash the task button in the NextSTART taskbar (or play the "attention" effect for the associated icon in a dock) when the application requires it. In the case of the NextSTART taskbar, after a little bit of flashing the button in the NextSTART taskbar stops flashing and assumes the "selected" visual state so the user knows that particular application requires your attention.

By design (this happened with the Windows taskbar too) if the NextSTART taskbar (or a dock showing running applications) is hidden, when an application invokes this "look at me" mechanism the taskbar or dock will pop up and will not auto-hide again until the user acknowledges that the marked application(s) require(s) attention.

This is done by clicking on that application so that comes to the foreground (i.e. clicking on the actual window that invoked the "hey, I need you to look at me" mechanism) - at that point the button on the taskbar (or icon in the dock) will no longer be marked as requiring attention and the taskbar/dock will auto-hide again.

So, when you click on Outlook, the respective button in the NextSTART taskbar should no longer show the "selected" state and the taskbar should auto-hide itself again.

I just tested this mechanism here and it's working as intended.

So according to your description it seems like it is the small Outlook box that appears while it loads your profile that invokes the attention mechanism for a reason that probably only makes sense to Microsoft.

I can see something similar happening here with Outlook 2021, when I launch it the button in the NextSTART taskbar briefly turns blue/flashes, but then it immediately stops flashing (and the taskbar hides if I have the auto-hide option enabled).

The fact that it doesn't do that there is weird. So here is where we need more details: what version of Outlook, what version of Winstep Xtreme, what version of Windows, and does it happen with other applications or only Outlook (and if so, which) ?

Microsoft® Outlook® for Microsoft 365 MSO (Version 2408 Build 16.0.17928.20114) 64-bit
NextSTART version 24.4

Outlook is already at the foreground when the main window comes up. I tried clicking on the blue bar in NextSTART but that just minimizes Outlook. So I clicked it again and Outlook came back, the blue bar went away, and NextSTART hid itself. I suppose that's a quicker way than restarting NextSTART.