Fix NavigationView focus routing on mouse click and ScrollablePanel last-line clipping

Replace _navPaneHasFocus boolean with computed NavPaneHasFocus property
derived from _navScrollPanel.HasFocus (single source of truth). Previously,
mouse clicks bypassed the HasFocus setter and never updated _navPaneHasFocus,
causing arrow keys to scroll the panel instead of moving selection.

Expose FocusedContent on HorizontalGridControl. Refactor FocusNavPane and
FocusContentPanel to toggle HasFocus directly on both panels.

Fix ScrollablePanel CalculateContentHeight width mismatch: measure at full
width first, re-measure at viewport-2 only when content overflows (matching
the paint loop's 2-column scrollbar reservation). The previous -1 subtraction
caused phantom scrollbars and last-line clipping.

Author: nickprotop

SharpConsoleUI/Controls/HorizontalGridControl/HorizontalGridControl.cs

@@ -76,6 +76,11 @@ public partial class HorizontalGridControl : BaseControl, IInteractiveControl, I
 		private List<ColumnContainer> _columns = new List<ColumnContainer>();
 		private readonly object _gridLock = new();
 		private IInteractiveControl? _focusedContent;
+
+		/// <summary>
+		/// Gets the currently focused child control within the grid.
+		/// </summary>
+		public IInteractiveControl? FocusedContent => _focusedContent;
 		private bool _hasFocus;
 		private Dictionary<IInteractiveControl, ColumnContainer> _interactiveContents = new Dictionary<IInteractiveControl, ColumnContainer>();
 		private bool _interactiveContentsDirty = true;

SharpConsoleUI/Controls/NavigationView/NavigationView.Input.cs

@@ -53,7 +53,11 @@ public bool ProcessMouseEvent(MouseEventArgs args)
 		#region IInteractiveControl / IFocusableControl Implementation
 
 		private bool _hasFocus;
-		private bool _navPaneHasFocus;
+
+		/// <summary>
+		/// Whether the nav pane has focus — derived from the scroll panel's own focus state (single source of truth).
+		/// </summary>
+		private bool NavPaneHasFocus => _navScrollPanel.HasFocus;
 
 		/// <inheritdoc/>
 		public bool HasFocus
@@ -72,7 +76,6 @@ public bool HasFocus
 				}
 				else if (!value && hadFocus)
 				{
-					_navPaneHasFocus = false;
 					_grid.HasFocus = false;
 					LostFocus?.Invoke(this, EventArgs.Empty);
 				}
@@ -120,7 +123,7 @@ public void SetFocusWithDirection(bool focus, bool backward)
 		/// <inheritdoc/>
 		public bool ProcessKey(ConsoleKeyInfo key)
 		{
-			if (_navPaneHasFocus)
+			if (NavPaneHasFocus)
 				return ProcessNavPaneKey(key);
 			else
 				return ProcessContentPanelKey(key);
@@ -136,10 +139,13 @@ public void NotifyChildFocusChanged(IInteractiveControl child, bool hasFocus)
 			// The grid is our only child — propagate focus tracking upward
 			if (child == _grid || child is HorizontalGridControl)
 			{
-				if (hasFocus && !_hasFocus)
+				if (hasFocus)
 				{
-					_hasFocus = true;
-					GotFocus?.Invoke(this, EventArgs.Empty);
+					if (!_hasFocus)
+					{
+						_hasFocus = true;
+						GotFocus?.Invoke(this, EventArgs.Empty);
+					}
 				}
 				else if (!hasFocus && _hasFocus)
 				{
@@ -399,18 +405,17 @@ private void FireItemInvoked()
 
 		private void FocusNavPane()
 		{
-			_navPaneHasFocus = true;
-			// Remove focus from content panel
+			// Remove focus from content panel and set it on the nav scroll panel
 			if (_contentPanel is IFocusableControl fc)
 				fc.HasFocus = false;
-			_grid.HasFocus = false;
+			_navScrollPanel.HasFocus = true;
 			Container?.Invalidate(true);
 		}
 
 		private void FocusContentPanel()
 		{
-			_navPaneHasFocus = false;
-			// Set focus on the content panel
+			// Remove focus from nav panel and set it on the content panel
+			_navScrollPanel.HasFocus = false;
 			if (_contentPanel is IFocusableControl fc)
 				fc.HasFocus = true;
 			Container?.Invalidate(true);

SharpConsoleUI/Controls/ScrollablePanelControl/ScrollablePanelControl.Rendering.cs

@@ -477,16 +477,25 @@ private void DrawBottomBorder(CharacterBuffer buffer, int x, int y, int width, L
 
 		private int CalculateContentHeight(int viewportWidth, int maxHeight = 0)
 		{
-			int availableWidth = viewportWidth;
-			int maxH = maxHeight > 0 ? maxHeight : int.MaxValue;
+			// Measure at full width first, then re-measure at reduced width only if
+			// the content actually overflows and a scrollbar will appear.
+			int fullHeight = MeasureChildrenHeight(viewportWidth, maxHeight);
 
-			// Reserve space for scrollbar if we might need it
-			// This is an approximation - we'll recalculate if needed
-			if (_showScrollbar && _verticalScrollMode == ScrollMode.Scroll)
+			int viewportH = maxHeight > 0 ? maxHeight : _viewportHeight;
+			if (_showScrollbar && _verticalScrollMode == ScrollMode.Scroll && fullHeight > viewportH)
 			{
-				availableWidth = Math.Max(1, viewportWidth - 1);
+				int narrowWidth = Math.Max(1, viewportWidth - 2);
+				if (narrowWidth != viewportWidth)
+					return MeasureChildrenHeight(narrowWidth, maxHeight);
 			}
 
+			return fullHeight;
+		}
+
+		private int MeasureChildrenHeight(int availableWidth, int maxHeight)
+		{
+			int maxH = maxHeight > 0 ? maxHeight : int.MaxValue;
+
 			List<IWindowControl> calcSnapshot;
 			lock (_childrenLock) { calcSnapshot = new List<IWindowControl>(_children); }