Fix eraser tool for rectangles/circles + add shift modifier for perfect squares

[Copilot]

Eraser tool only removed polylines and lines, ignoring rectangles and circles. Rectangle tool lacked shift-to-constrain functionality present in circle tool.

Changes

Eraser Tool (EraserTool.cs)

• Added bounds intersection detection for Rectangle and Ellipse shapes using IntersectsWith()
• Added NaN guards for Canvas.GetLeft/GetTop to handle uninitialized attached properties

Rectangle Tool (RectangleTool.cs)

• Added shift key detection in OnMouseMove and FinishRectangle to constrain to perfect square
• Uses Math.Min(width, height) to fit square within dragged bounds (differs from circle tool's Math.Max approach per issue spec)

// Eraser now handles all shape types
else if (element is System.Windows.Shapes.Rectangle rectangle)
{
    double left = Canvas.GetLeft(rectangle);
    double top = Canvas.GetTop(rectangle);
    
    if (!double.IsNaN(left) && !double.IsNaN(top) && ...)
    {
        Rect shapeRect = new Rect(left, top, rectangle.Width, rectangle.Height);
        if (eraserRect.IntersectsWith(shapeRect))
            shouldErase = true;
    }
}

// Rectangle tool constrains to square when shift held
bool isShiftPressed = Keyboard.IsKeyDown(Key.LeftShift) || Keyboard.IsKeyDown(Key.RightShift);
if (isPerfectSquare)
{
    double size = Math.Min(width, height);
    width = size;
    height = size;
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Eraser Tool Cannot Erase Rectangles/Circles + Add Perfect Square Modifier</issue_title>
<issue_description>## Bug/Enhancement: Eraser Tool Shape Support + Rectangle Perfect Squares

Issues

1. Eraser Tool Cannot Erase Rectangles and Circles (BUG)

Description:
The Eraser tool currently only erases Polylines (pen strokes) and Lines (straight line tool), but does not erase Rectangle or Circle shapes. This creates an inconsistent user experience where some drawn elements cannot be removed.

Current Behavior:

• ✅ Eraser removes Polylines (Pen tool strokes)
• ✅ Eraser removes Lines (Line tool)
• ❌ Eraser does NOT remove Rectangles (Rectangle tool)
• ❌ Eraser does NOT remove Circles (Circle tool)

Expected Behavior:
All drawn shapes should be erasable with the Eraser tool, regardless of which tool created them.

Impact:

• Users cannot clean up Rectangle/Circle drawings without clearing the entire canvas (R key)
• Inconsistent tool behavior reduces usability
• Workaround: Must use Undo or clear entire canvas

2. Rectangle Tool - Add Perfect Square Modifier (ENHANCEMENT)

Description:
Add Shift modifier to Rectangle tool to constrain proportions and create perfect squares.

Proposed Behavior:

• Without Shift: Draw any rectangle (current behavior)
• With Shift held: Constrain to perfect square (width = height)
• Similar to how image editing tools work (Photoshop, GIMP, etc.)

User Interaction:

1. Press U to activate Rectangle tool
2. Click first corner
3. Hold Shift while moving mouse - preview shows perfect square
4. Click second corner to finalize square

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Proposed Solutions

Part 1: Fix Eraser Tool for Rectangles and Circles

Root Cause:
The EraserTool.cs likely only checks for Polyline and Line shape types in its intersection detection logic.

Implementation:

Update Src/GhostDraw/Tools/EraserTool.cs:

// Current code probably looks like:
if (child is Polyline polyline)
{
    // Check intersection with polyline
}
else if (child is Line line)
{
    // Check intersection with line
}

// Add these cases:
else if (child is Rectangle rectangle)
{
    // Check if eraser bounds intersects with rectangle bounds
    Rect eraserRect = new Rect(position.X - radius, position.Y - radius, 
                                radius * 2, radius * 2);
    Rect shapeRect = new Rect(
        Canvas.GetLeft(rectangle), 
        Canvas.GetTop(rectangle),
        rectangle.Width, 
        rectangle.Height
    );
    
    if (eraserRect.IntersectsWith(shapeRect))
    {
        canvas.Children.Remove(rectangle);
    }
}
else if (child is Ellipse ellipse)
{
    // Check if eraser bounds intersects with ellipse bounds
    Rect eraserRect = new Rect(position.X - radius, position.Y - radius, 
                                radius * 2, radius * 2);
    Rect ellipseRect = new Rect(
        Canvas.GetLeft(ellipse), 
        Canvas.GetTop(ellipse),
        ellipse.Width, 
        ellipse.Height
    );
    
    if (eraserRect.IntersectsWith(ellipseRect))
    {
        canvas.Children.Remove(ellipse);
    }
}

Files to Modify:

• Src/GhostDraw/Tools/EraserTool.cs - Add Rectangle and Ellipse handling

Testing:

• Create unit tests in Tests/GhostDraw.Tests/EraserToolTests.cs
• Verify eraser removes rectangles when cursor overlaps
• Verify eraser removes circles when cursor overlaps
• Ensure existing Polyline/Line erasing still works

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Part 2: Add Shift Modifier for Perfect Squares

Implementation:

Update Src/GhostDraw/Tools/RectangleTool.cs:

public void OnMouseMove(Point position, Canvas canvas, MouseButtonState leftButtonState)
{
    if (_isCreatingRectangle && _currentRectangle != null && _rectangleStartPoint.HasValue)
    {
        Point start = _rectangleStartPoint.Value;
        double width = Math.Abs(position.X - start.X);
        double height = Math.Abs(position.Y - start.Y);
        
        // Check if Shift is held for perfect square
        bool isShiftHeld = Keyboard.IsKeyDown(Key.LeftShift) || Keyboard.IsKeyDown(Key.RightShift);
        
        if (isShiftHeld)
        {
            // Constrain to square - use smaller dimension
            double size = Math.Min(width, height);
            width = size;
            height = size;
        }
        
        _currentRectangle.Width = width;
        _currentRectangle.Height = height;
        
        // Position rectangle at correct corner
        Canvas.SetLeft(_currentRectangle, Math.Min(start.X, position.X));
        Canvas.SetTop(_currentRectangle, Math.Min(start.Y, position.Y));
    }
}

Files to Modify:

• Src/GhostDraw/Tools/RectangleTool.cs - Add Shift key detection and constraint logic

Documentation Updates:

• README.md - Mention Shift modifier for squares
• Views/OverlayWindow.xaml - Update F1 help overlay
• CHANGELOG.md - Document enhance...

• Fixes Eraser Tool Cannot Erase Rectangles/Circles + Add Perfect Square Modifier #28

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