Passing through moveFocus keys when moving to another pane failed (#10806)

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## Summary of the Pull Request
Implementation of #6219 with a small tweak, not just passing the keys when no panes are present, but passing on the keys when there is no other pane to move to. This enables another usecase: 2 panes in terminal split vertically; in one of these panes running tmux with two panes that are split horizontally. This allows the user to still navigate between tmux panes even though they have terminal panes open.

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## References
Not that I know of

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## PR Checklist
* [x] Closes #6219
* [x] CLA signed.
* [x] Tests added/passed
* [x] Documentation updated. I don't think that's necessary
* [x] Schema updated. N/A
* [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan.

<!-- Provide a more detailed description of the PR, other things fixed or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments
Implementation by propagating the boolean indicating success of moving focus all the way to the action handler, where this result will determine whether the action will be considered handled or not. When the action is not handled, the keychord will be propagated to the terminal.

<!-- Describe how you validated the behavior. Add automated tests wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed
Manual testing; all relevant unit tests still work

src/cascadia/TerminalApp/AppActionHandlers.cpp

@@ -307,8 +307,11 @@ namespace winrt::TerminalApp::implementation
             }
             else
             {
-                _MoveFocus(realArgs.FocusDirection());
-                args.Handled(true);
+                // Mark as handled only when the move succeeded (e.g. when there
+                // is a pane to move to), otherwise mark as unhandled so the
+                // keychord can propagate to the terminal (GH#6129)
+                const auto moveSucceeded = _MoveFocus(realArgs.FocusDirection());
+                args.Handled(moveSucceeded);
             }
         }
     }

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -1123,14 +1123,16 @@ namespace winrt::TerminalApp::implementation
     // Arguments:
     // - direction: The direction to move the focus in.
     // Return Value:
-    // - <none>
-    void TerminalPage::_MoveFocus(const FocusDirection& direction)
+    // - Whether changing the focus succeeded. This allows a keychord to propagate
+    //   to the terminal when no other panes are present (GH#6219)
+    bool TerminalPage::_MoveFocus(const FocusDirection& direction)
     {
         if (const auto terminalTab{ _GetFocusedTabImpl() })
         {
             _UnZoomIfNeeded();
-            terminalTab->NavigateFocus(direction);
+            return terminalTab->NavigateFocus(direction);
         }
+        return false;
     }
 
     // Method Description:

src/cascadia/TerminalApp/TerminalPage.h

@@ -234,7 +234,7 @@ namespace winrt::TerminalApp::implementation
 
         void _SelectNextTab(const bool bMoveRight, const Windows::Foundation::IReference<Microsoft::Terminal::Settings::Model::TabSwitcherMode>& customTabSwitcherMode);
         bool _SelectTab(uint32_t tabIndex);
-        void _MoveFocus(const Microsoft::Terminal::Settings::Model::FocusDirection& direction);
+        bool _MoveFocus(const Microsoft::Terminal::Settings::Model::FocusDirection& direction);
         void _MovePane(const Microsoft::Terminal::Settings::Model::FocusDirection& direction);
 
         winrt::Microsoft::Terminal::Control::TermControl _GetActiveControl();

src/cascadia/TerminalApp/TerminalTab.cpp

@@ -481,23 +481,24 @@ namespace winrt::TerminalApp::implementation
     // Arguments:
     // - direction: The direction to move the focus in.
     // Return Value:
-    // - <none>
-    void TerminalTab::NavigateFocus(const FocusDirection& direction)
+    // - Whether changing the focus succeeded. This allows a keychord to propagate
+    //   to the terminal when no other panes are present (GH#6219)
+    bool TerminalTab::NavigateFocus(const FocusDirection& direction)
     {
         if (direction == FocusDirection::Previous)
         {
             if (_mruPanes.size() < 2)
             {
-                return;
+                return false;
             }
             // To get to the previous pane, get the id of the previous pane and focus to that
-            _rootPane->FocusPane(_mruPanes.at(1));
+            return _rootPane->FocusPane(_mruPanes.at(1));
         }
         else
         {
             // NOTE: This _must_ be called on the root pane, so that it can propagate
             // throughout the entire tree.
-            _rootPane->NavigateFocus(direction);
+            return _rootPane->NavigateFocus(direction);
         }
     }
 

src/cascadia/TerminalApp/TerminalTab.h

@@ -52,7 +52,7 @@ namespace winrt::TerminalApp::implementation
 
         void ResizeContent(const winrt::Windows::Foundation::Size& newSize);
         void ResizePane(const winrt::Microsoft::Terminal::Settings::Model::ResizeDirection& direction);
-        void NavigateFocus(const winrt::Microsoft::Terminal::Settings::Model::FocusDirection& direction);
+        bool NavigateFocus(const winrt::Microsoft::Terminal::Settings::Model::FocusDirection& direction);
         void MovePane(const winrt::Microsoft::Terminal::Settings::Model::FocusDirection& direction);
         bool FocusPane(const uint32_t id);