Some applications doesn't have the tab view with the lines connecting to the outline of the content "box", like the one seem in Kate:
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**Examples of apps that aren't following this tab view style**
Dolphin:
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Konsole (maybe the active tab could pick the color of the shell background color, because an outline without any margins could look weird):
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KSysGuard:
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**Outline color inconsistency:**
Also, in Kate, the active tab bar makes changes the outline color, which doesn't happen in the tab view of Settings > Window Behavior, or the Activity settings (in System Settings):
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