FIX: Lock axes bounds in plot_joint to prevent shrinking on cursor motion (closes #14317)

[AkhilG-exe]

Why

evoked.plot_joint() figures shrink to zero once the cursor moves (or hovers) over them (closes #14317).

The joint plot is a layout="constrained" figure. The connection lines between the time series and the topomaps are drawn as ConnectionPatch(..., clip_on=False) artists added to the butterfly (ts_ax) axes. Matplotlib's constrained layout computes each axes' margins from Axes.get_tightbbox(for_layout_only=True), which includes every artist not fully clipped to the axes. Because these lines poke outside ts_ax and never move with it, every redraw (which cursor motion triggers through the interactive hover/_BlitManager path) measures a larger margin than the last, and the solver turns that growth back into a smaller axes — a feedback loop collapsing all axes to zero.

Reproduces with a plain sequence of fig.canvas.draw() calls:

redraws	1	2	3	10
butterfly axes height (before)	0.481	0.452	0.423	0.003
butterfly axes height (after)	0.507	0.507	0.507	0.507

What

• mne/viz/evoked.py: call con.set_in_layout(False) on the connection lines so constrained layout ignores them. This is the matplotlib-idiomatic mechanism for artists that intentionally leave their axes; rendering is unchanged and the initial layout is essentially the same.

• mne/viz/tests/test_topo.py: regression assertions added to the existing test_plot_joint — the largest axes' position must be unchanged after repeated redraws.

• doc/changes/dev/14319.bugfix.rst: changelog fragment.

• doc/changes/names.inc: new contributor entry.

Test plan

• pytest mne/viz/tests/test_topo.py mne/viz/tests/test_evoked.py → 25 passed

• test_plot_joint fails without the fix (regression) and passes with it

• TFR joint-plot tests (test_tfr_plot_joint, test_tfr_plot_joint_doesnt_modify) → 16 passed (TFR path was already safe: its ConnectionPatch is attached to the figure, not an axes)

• ruff check / ruff format --check clean on modified files

• Before/after visual verification: pre-fix renders drift by 122,985 pixels over 9 redraws; post-fix renders are bit-identical (0 pixels).

Disclosure per the AI-assistance policy: this change was developed with assistance from an AI coding agent (opencode), which diagnosed the constrained-layout feedback loop, while I implemented the one-line fix, and drafted the regression test. I reviewed and verified all changes. as well.

[CarinaFo]

Thanks for the quick bugfix, looks good. I verified the bug and the fix on Windows 11.