Prevent multiple settings tabs from being persisted (#17169)

Re-add some machinery to special case settings tabs. When we're going to
persist a tab that only has a single settings pane in it, we'll now
promote that to the first-class "openSettings" action. This will allow
our other code in TerminalPage to route multiple settings tabs all to
the same tab.

This of course doesn't stop you from opening multiple settings tabs with
`{ "command": {"action": "newTab", "type": "settings"} }` actions. If we
did that, then that would prevent someone from having a settings pane in
the first pane, and a terminal to the right.

Closes #17070

src/cascadia/TerminalApp/TerminalTab.cpp

@@ -449,9 +449,23 @@ namespace winrt::TerminalApp::implementation
 
  {
  ActionAndArgs newTabAction{};
+ INewContentArgs newContentArgs{ state.firstPane->GetTerminalArgsForPane(kind) };
+
+ // Special case here: if there was one pane (which results in no actions
+ // being generated), and it was a settings pane, then promote that to an
+ // open settings action. The openSettings action itself has additional machinery
+ // to prevent multiple top-level settings tabs.
+ const auto wasSettings = state.args.empty() &&
+ (newContentArgs && newContentArgs.Type() == L"settings");
+ if (wasSettings)
+ {
+ newTabAction.Action(ShortcutAction::OpenSettings);
+ newTabAction.Args(OpenSettingsArgs{ SettingsTarget::SettingsUI });
+ return std::vector<ActionAndArgs>{ std::move(newTabAction) };
+ }
+
  newTabAction.Action(ShortcutAction::NewTab);
- NewTabArgs newTabArgs{ state.firstPane->GetTerminalArgsForPane(kind) };
- newTabAction.Args(newTabArgs);
+ newTabAction.Args(NewTabArgs{ newContentArgs });
 
  state.args.emplace(state.args.begin(), std::move(newTabAction));
  }