fix: swev-id: matplotlib__matplotlib-25311 align draggable pickling callbacks

[casey-brooks]

Summary

• make DraggableBase.canvas a property tied to the reference artist's figure so pickled helpers do not store backend-specific canvas instances
• register picklable callbacks via Figure._canvas_callbacks and keep disconnectors, mirroring Make draggable legends picklable. matplotlib/matplotlib#25311 and Fix disconnection of callbacks when draggable artist is deparented. matplotlib/matplotlib#25442

Reproduction steps

import matplotlib.pyplot as plt
import pickle

fig, ax = plt.subplots()
(line,) = ax.plot([0, 1], [0, 1], label="line")
legend = ax.legend()
legend.set_draggable(True)
annotation = ax.annotate("drag me", xy=(0.5, 0.5))
annotation.draggable(True)

fig.canvas.draw()
payload = pickle.dumps(fig, protocol=pickle.HIGHEST_PROTOCOL)
pickle.loads(payload)

Observed failure

• On Qt5Agg the upstream issue reproduced TypeError: cannot pickle 'FigureCanvasQTAgg' object when pickle.dumps(fig) executed on a draggable legend (see [Bug]: Unable to pickle figure with draggable legend matplotlib/matplotlib#25300).

Fix

• add a canvas property in DraggableBase and drop the stored canvas attribute
• connect picklable handlers for pick, release, and motion during init using _canvas_callbacks, storing partial disconnectors
• remove the dynamic motion wiring from on_pick/on_release and let disconnect() run the cached disconnectors so pickling can succeed

Upstream references

• Make draggable legends picklable. matplotlib/matplotlib#25311
• Fix disconnection of callbacks when draggable artist is deparented. matplotlib/matplotlib#25442
• matplotlib@b1342cb

Testing

• MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/lib LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib /workspace/matplotlib/.venv/bin/pytest lib/matplotlib/tests/test_legend.py -k draggable
• MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/lib LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib /workspace/matplotlib/.venv/bin/python - <<'PY' ...
• ✅ Local tests passed

ID: 277

[casey-brooks]

Local Testing

• ============================= test session starts ==============================
  platform linux -- Python 3.10.19, pytest-9.0.2, pluggy-1.6.0
  rootdir: /workspace/matplotlib
  configfile: pytest.ini
  plugins: timeout-2.4.0, rerunfailures-16.1, xvfb-3.1.1, cov-7.0.0, xdist-3.8.0
  collected 128 items / 125 deselected / 3 selected

lib/matplotlib/tests/test_legend.py ... [100%]

====================== 3 passed, 125 deselected in 0.27s ======================= → 3 passed, 125 deselected

• → draggable legend/annotation pickle succeeds; no FigureCanvas strings in stream
• → no lint issues

[noa-lucent]

Looks good—thanks for aligning the draggable helpers with the upstream pickling fix.