Prevent splitting panes into 0 width/height #2401 (#2450)

Fixes a crash that can occur when splitting pane that was so small that the target panes would have a width/height of 0, causing DxRenderer to fail when creating the device resources.

This PR prevents both the call to `App::AddHorizontal/VerticalSplit` and the creation of the `TermControl` if the split would fail.

Closes #2401

## Details

`App::_SplitPane` calls `focusedTab->CanAddHorizontalSplit/CanAddHorizontalSplit` before it initializes the `TermControl` to avoid having to deal with the cleanup. If a split cannot occur, it will simply return. 

**Question: Should we beep or something here?**

It then follows the same naming/flow style as the split operation, so: `Tab::CanAddHorizontalSplit -> Pane::CanSplitHorizontal ->Pane::_CanSplit`. The public pane methods will handle leaf/child the same as the current Split methods.

`_CanSplit` reuses existing logic like `_root.GetActualWidth/Height`, `Pane::_GetMinSize`, and the `Half` constant.

## Validation Steps Performed

1. Open a new tab
2. Attempt to split horizontally/vertically more than 6-8 times

Success: Pane will will eventually stop splitting rather than crashing the process.

src/cascadia/TerminalApp/App.cpp

@@ -1494,11 +1494,19 @@ namespace winrt::TerminalApp::implementation
 
         const auto controlConnection = _CreateConnectionFromSettings(realGuid, controlSettings);
 
-        TermControl newControl{ controlSettings, controlConnection };
-
         const int focusedTabIndex = _GetFocusedTabIndex();
         auto focusedTab = _tabs[focusedTabIndex];
 
+        const auto canSplit = splitType == Pane::SplitState::Horizontal ? focusedTab->CanAddHorizontalSplit() :
+                                                                          focusedTab->CanAddVerticalSplit();
+
+        if (!canSplit)
+        {
+            return;
+        }
+
+        TermControl newControl{ controlSettings, controlConnection };
+
         // Hookup our event handlers to the new terminal
         _RegisterTerminalEvents(newControl, focusedTab);
 

src/cascadia/TerminalApp/Pane.cpp

@@ -777,6 +777,31 @@ void Pane::_ApplySplitDefinitions()
     }
 }
 
+// Method Description:
+// - Determines whether the pane can be split vertically
+// Arguments:
+// - splitType: what type of split we want to create.
+// Return Value:
+// - True if the pane can be split vertically. False otherwise.
+bool Pane::CanSplitVertical()
+{
+    if (!_IsLeaf())
+    {
+        if (_firstChild->_HasFocusedChild())
+        {
+            return _firstChild->CanSplitVertical();
+        }
+        else if (_secondChild->_HasFocusedChild())
+        {
+            return _secondChild->CanSplitVertical();
+        }
+
+        return false;
+    }
+
+    return _CanSplit(SplitState::Vertical);
+}
+
 // Method Description:
 // - Vertically split the focused pane in our tree of panes, and place the given
 //   TermControl into the newly created pane. If we're the focused pane, then
@@ -806,6 +831,31 @@ void Pane::SplitVertical(const GUID& profile, const TermControl& control)
     _Split(SplitState::Vertical, profile, control);
 }
 
+// Method Description:
+// - Determines whether the pane can be split horizontally
+// Arguments:
+// - splitType: what type of split we want to create.
+// Return Value:
+// - True if the pane can be split horizontally. False otherwise.
+bool Pane::CanSplitHorizontal()
+{
+    if (!_IsLeaf())
+    {
+        if (_firstChild->_HasFocusedChild())
+        {
+            return _firstChild->CanSplitHorizontal();
+        }
+        else if (_secondChild->_HasFocusedChild())
+        {
+            return _secondChild->CanSplitHorizontal();
+        }
+
+        return false;
+    }
+
+    return _CanSplit(SplitState::Horizontal);
+}
+
 // Method Description:
 // - Horizontally split the focused pane in our tree of panes, and place the given
 //   TermControl into the newly created pane. If we're the focused pane, then
@@ -834,6 +884,40 @@ void Pane::SplitHorizontal(const GUID& profile, const TermControl& control)
     _Split(SplitState::Horizontal, profile, control);
 }
 
+// Method Description:
+// - Determines whether the pane can be split.
+// Arguments:
+// - splitType: what type of split we want to create.
+// Return Value:
+// - True if the pane can be split. False otherwise.
+bool Pane::_CanSplit(SplitState splitType)
+{
+    const bool changeWidth = _splitState == SplitState::Vertical;
+
+    const Size actualSize{ gsl::narrow_cast<float>(_root.ActualWidth()),
+                           gsl::narrow_cast<float>(_root.ActualHeight()) };
+
+    const Size minSize = _GetMinSize();
+
+    if (splitType == SplitState::Vertical)
+    {
+        const auto widthMinusSeparator = actualSize.Width - PaneSeparatorSize;
+        const auto newWidth = widthMinusSeparator * Half;
+
+        return newWidth > minSize.Width;
+    }
+
+    if (splitType == SplitState::Horizontal)
+    {
+        const auto heightMinusSeparator = actualSize.Height - PaneSeparatorSize;
+        const auto newHeight = heightMinusSeparator * Half;
+
+        return newHeight > minSize.Height;
+    }
+
+    return false;
+}
+
 // Method Description:
 // - Does the bulk of the work of creating a new split. Initializes our UI,
 //   creates a new Pane to host the control, registers event handlers.

src/cascadia/TerminalApp/Pane.h

@@ -49,7 +49,10 @@ public:
     bool ResizePane(const winrt::TerminalApp::Direction& direction);
     bool NavigateFocus(const winrt::TerminalApp::Direction& direction);
 
+    bool CanSplitHorizontal();
     void SplitHorizontal(const GUID& profile, const winrt::Microsoft::Terminal::TerminalControl::TermControl& control);
+
+    bool CanSplitVertical();
     void SplitVertical(const GUID& profile, const winrt::Microsoft::Terminal::TerminalControl::TermControl& control);
 
     void Close();
@@ -79,6 +82,7 @@ private:
     bool _HasFocusedChild() const noexcept;
     void _SetupChildCloseHandlers();
 
+    bool _CanSplit(SplitState splitType);
     void _Split(SplitState splitType, const GUID& profile, const winrt::Microsoft::Terminal::TerminalControl::TermControl& control);
     void _CreateRowColDefinitions(const winrt::Windows::Foundation::Size& rootSize);
     void _CreateSplitContent();

src/cascadia/TerminalApp/Tab.cpp

@@ -203,6 +203,15 @@ void Tab::Scroll(const int delta)
     });
 }
 
+// Method Description:
+// - Determines whether the focused pane has sufficient space to be split vertically.
+// Return Value:
+// - True if the focused pane can be split horizontally. False otherwise.
+bool Tab::CanAddVerticalSplit()
+{
+    return _rootPane->CanSplitVertical();
+}
+
 // Method Description:
 // - Vertically split the focused pane in our tree of panes, and place the
 //   given TermControl into the newly created pane.
@@ -216,6 +225,15 @@ void Tab::AddVerticalSplit(const GUID& profile, TermControl& control)
     _rootPane->SplitVertical(profile, control);
 }
 
+// Method Description:
+// - Determines whether the focused pane has sufficient space to be split horizontally.
+// Return Value:
+// - True if the focused pane can be split horizontally. False otherwise.
+bool Tab::CanAddHorizontalSplit()
+{
+    return _rootPane->CanSplitHorizontal();
+}
+
 // Method Description:
 // - Horizontally split the focused pane in our tree of panes, and place the
 //   given TermControl into the newly created pane.

src/cascadia/TerminalApp/Tab.h

@@ -19,7 +19,9 @@ public:
     void SetFocused(const bool focused);
 
     void Scroll(const int delta);
+    bool CanAddVerticalSplit();
     void AddVerticalSplit(const GUID& profile, winrt::Microsoft::Terminal::TerminalControl::TermControl& control);
+    bool CanAddHorizontalSplit();
     void AddHorizontalSplit(const GUID& profile, winrt::Microsoft::Terminal::TerminalControl::TermControl& control);
 
     void UpdateFocus();