XnView MP will open an image in a new tab, and the tab-to-window connection for that is how it needs to be:
Note that the active tab is both in contact with the window and shares its color. Moreover, the inactive tabs are clearly inactive, being much darker than the active tab. This is how tabs should work! (Don't get me started on those stupid button-style tabs that are all the rage right now
)The other tabbed interfaces in XnView MP are in need of improvement though.
The Source panels window is the "big one" in need of this.
There, the active tab is neither the same color (exception: Default theme) nor connected to its window:
In the Grey theme, and perhaps in others, this issue is compounded by the fact that the inactive tabs are closer in color to the window than the active one is.
So this should be refined (please excuse my horrible photoshopping skills):
The images above use vertical tabs, but the issue is the same for horizontal tabs.
Note that I lowered the tabs in the mock-up as well. This is to avoid the Folders tab tucking itself behind the toolbar of the Catalog filter tab, when this latter is active. This too creates confusion, suggesting that that bar is there all the time. I'm open to suggestions for solving that issue.
The other area where this is an issue is in the Info panels when both "Preview as tab" and "Use tabs" are checked at View > Info panels. This is more problematic though, because the user can change the preview background color. What I might suggest there is a small internal frame that shares the active tab color. That might be problematic however for people who want every pixel of the preview window for the image?? Brainstorming is welcome for that as well.
As always, thank you for your consideration.