Add basic shape tools to the example drawing area

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Seit 08.11.2026.

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Beschreibung

Observed behavior

The Example drawing area introduced in PR #262 allows candidates to illustrate examples during coding interviews without hiding the code editor. It currently supports freehand drawing, erasing, undo, clear, color selection, and text labels, but does not provide tools for drawing basic geometric shapes.

Candidates must manually draw rectangles, circles, and straight lines using the pen tool, making it difficult to create consistent diagrams when explaining data structures, algorithms, or relationships between elements.

Expected behavior

The Example drawing area should support three additional drawing tools:

  • Rectangle: Draw a rectangle between two points.
  • Ellipse: Draw a circle or ellipse within the dragged bounds.
  • Line: Draw a straight line between two points.

Candidates should be able to preview a shape while dragging and complete it when releasing the pointer.

Each completed shape should behave as a single undoable operation, reuse the existing color selection, and remain compatible with recording, replay, and reconnection.

Reproduction

  1. Start a Coding interview with Example drawing enabled.
  2. Expand the Example drawing area below the code editor.
  3. Observe that the toolbar provides pen, eraser, text, undo, clear, and color selection.
  4. Attempt to draw a rectangle, circle, or straight line.
  5. Observe that these shapes must be drawn manually using the pen tool.

Initial analysis

The drawing model and rendering logic are implemented in web/whiteboard.js, with corresponding tests in tests/browser/whiteboard.test.js. The existing drawing operations support undo, clearing, and replay through a shared drawing model.

PR #523 introduced structured text operations shared by Whiteboard and Example drawing. The proposed shape tools should reuse the existing drawing infrastructure where possible.

The implementation would need to consider:

  • Toolbar controls: Add rectangle, ellipse, and line tools alongside the existing drawing controls.
  • Pointer handling: Track the initial pointer position and current position to determine shape geometry.
  • Drawing preview: Render temporary shapes during dragging without adding intermediate previews to the operation history.
  • Operation history: Commit each completed shape as a single undoable operation.
  • Recording and replay: Ensure that completed shapes are preserved and rendered correctly.
  • Reconnection: Retain completed drawings after reconnecting.

The operation representation should minimize changes to the existing drawing protocol while preserving compatibility with recording, replay, and reconnection.

Proposed change

Extend the Example drawing area with three basic shape tools: Rectangle, Ellipse, and Line.

Selecting a shape tool should allow candidates to press, drag, and release the pointer to create the corresponding shape. A temporary preview should be displayed during dragging, and the completed shape should be committed to the drawing history when the pointer is released.

The implementation should preserve existing pen, eraser, text, undo, clear, color selection, recording, replay, and reconnection behavior.

Validation plan

Add browser regression tests covering:

  • Rectangle, ellipse, and line creation using pointer interactions.
  • Shape geometry when dragging in different directions.
  • Shape preview and final rendering.
  • Color selection for newly created shapes.
  • Undo and clear behavior for completed shapes.
  • Recording and replay of drawings containing geometric shapes.
  • Preservation of completed shapes after reconnection.
  • Regression coverage for existing pen, eraser, and text tools.

Tests should verify the actual drawing output rather than only checking whether toolbar buttons or canvas elements are visible.

Run ./scripts/test.sh and report the results, including any skipped checks.

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Rust
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