Only enable maximize terminal button in split header bar when usable

Disable the maximize terminal button when there is only one split
in a tab.

BUG: 435029

src/widgets/TerminalHeaderBar.cpp

@@ -278,12 +278,14 @@ void TerminalHeaderBar::applyVisibilitySettings()
 {
     auto *settings = KonsoleSettings::self();
     auto toVisibility = settings->splitViewVisibility();
+    const bool singleTerminalView = (getTopLevelSplitter()->findChildren<TerminalDisplay *>().count() == 1); 
     switch (toVisibility) {
     case KonsoleSettings::AlwaysShowSplitHeader:
+        m_toggleExpandedMode->setDisabled(singleTerminalView);
         setVisible(true);
         break;
     case KonsoleSettings::ShowSplitHeaderWhenNeeded: {
-        const bool visible = !(getTopLevelSplitter()->findChildren<TerminalDisplay *>().count() == 1);
+        const bool visible = !(singleTerminalView);
         setVisible(visible);
     } break;
     case KonsoleSettings::AlwaysHideSplitHeader:
@@ -293,4 +295,5 @@ void TerminalHeaderBar::applyVisibilitySettings()
     }
 }
 
+
 }