PR summary

I noticed some visual drift while working on evoked.animate_topomap code in MNE-python, whittled down to MWE (forced full-repaint exposes the full Agg canvas to show the bug more obviously):

import matplotlib.pyplot as plt
from matplotlib.animation import FuncAnimation

fig, ax = plt.subplots(figsize=(4, 2), dpi=100, layout="constrained")
ax.spines["top"].set_color("white")
ln = ax.axvline(0, color="r", lw=3)

def animate(i):
    ln.set_xdata([i / 199] * 2)
    if i % 20 == 0:
        fig.canvas.update()          # full repaint: buffer -> screen
    return [ln]

anim = FuncAnimation(fig, animate, frames=200, interval=10, blit=True, cache_frame_data=False)
fig.canvas.draw()  # force it to settle geometry
assert ax.bbox.y1 % 1, f"need a fractional Axes top edge, got {ax.bbox.y1}"
plt.show(block=True)

gives this for main, with just the copy-from-bbox fix, then with that plus the Qt fix:

main

Just copy-to-bbox fix

copy-to-box plus Qt (this PR's state)

I then refactored the code into a shared helper, and searched for other places for likely truncation errors, and fixed those too (gtk3agg, wxagg, cairo). Hopefully everything is consistently applied across backends now 🤞 Happy to reduce scope if you want, this is a (perhaps overly optimistic) "let's try to get everything consistent at once" push!

Related to #8837 (sadly, doesn't close because that issue tracks a second bug about GUIs being "unable to render the image with a non-integer origin").

Similar class of fix that #14461 made in Tk, and refactors that code to use a shared helper (DRY) to perhaps/hopefully address @anntzer comment there:

we should specify somewhere how rounding of bboxes passed to blit() (and to copy_from_bbox()) works

AI Disclosure

Claude Opus 5 was used to examine the issue and draft the changes. I reviewed and iterated (a lot) with it on the code (DRY helper, scoping backends, interactively testing, verifying fixes, etc.), and believe I ended up with the changes I likely would have made if I sat down for many hours working on this alone.

PR quality check

• Use an expressive title, e.g. "Fix title font property precedence"
• New and changed code is tested
• Plotting related features are demonstrated in an example (too subtle I think, but MWE above)
• New features and API changes have release notes
• Documentation complies with general and docstring guidelines (not a doc change)