current canvas out of sync after page reload

[gagern]

While debugging some other issue, I've encountered an exception in CanvasManagerModel.processCommand caused by this.currentCanvas being undefined.

Steps to reproduce:

1. Start new notebook.
2. Enter and run cell from ipycanvas import Canvas, hold_canvas
3. Enter and run cell canvas = Canvas(width=100, height=100)
4. Enter and run cell canvas.fill_rect(10, 10, 20, 20)
5. Re-load browser window
6. Optional: Enable pause on exception in developer tools
7. Re-execute the last cell

Expected behavior:
Command executed cleanly, or a reasonably understandable exception raised in Python code and surfaced in the UI.

Actual behavior:
Error thrown: Cannot read properties of undefined (reading 'fillRect')

Hypothesis:

I believe this is most likely due to a mismatch of state. The Python _CanvasManager._current_canvas refers to the last used canvas, but on page reload the JavaScript CanvasManagerModel gets re-created and there is no machinery in place (as far as I can tell) to sync the current canvas between the two.

Potential solutions:

• Have the CanvasManagerModel widget constructor signal its Python counterpart to invalidate its _current_canvas_ state. That way the next draw operation would sync state again.
• Use some IPyWidgets machinery to sync the current canvas state from Python to JavaScript upon widget recreation.

I don't know enough about IPyWidgets to judge which of these approaches to prefer, or how the latter would look in more detail.