PNG export, clipboard copy and a right-click save menu

[CSSFrancis]

Why

exportPNG has composited a full figure PNG for a while, but nothing could
reach it — it is exposed only on mount()'s handle, the postMessage protocol,
or Playwright, and anywidget discards render()'s return value. No Python entry
point, no UI, undocumented.

What

Hover a figure for a ⤓ badge in its corner, or right-click any plot (gutters
and colourbar included — the overlay canvas does not cover those):

• This panel — Copy image Ctrl+C, Save PNG, Save full view, Save at native resolution
• Whole figure — Copy, Save PNG, Save full view
• Theme — Current / Light / Dark, sticky per figure

The badge matters because JupyterLab, PyCharm and VS Code install their own
contextmenu and keyboard handlers and may swallow a right-click or Cmd+C
before the figure sees it. A test installs a capture-phase contextmenu
swallower, asserts right-click is dead, then asserts the badge still works.
The badge is revealed by real pointer movement, not mouseenter, so hover
chrome cannot leak into the visual baselines or the gallery thumbnails.

fig.savefig("paper.png", theme="light", scale=2)
fig.savefig("data.png", source="native", panel=plot)   # 1:1 with the data

Hosts add their own formats with handle.registerExportAction(...).

source	Output
view	As displayed — zoom, pan, contrast
full	Whole data extent at panel resolution
native	One output pixel per data pixel, decorations redrawn at that size

Two save entries. Save PNG… downloads with no permission prompt.
Save as… opens a real system dialog via showSaveFilePicker — listed only
where the browser supports it, because it hands the page a persistent writable
handle and so costs Chrome's "this site can see edits you make" prompt. Too
much for a plain save; right when the user asked to choose a folder.

A dialog the user closed and one that never opened both reject with
AbortError, so the name cannot separate them — one that never rendered comes
back in well under 250 ms, which is the discriminator. Without that, Save would
silently do nothing in headless and in webviews that stub the API.

panelId changes only the origin and extent, so a panel export is exactly the
matching sub-rectangle of the figure export. Theme swap, view reset, native
resize, composite and restore all run in one synchronous task, so no
intermediate state paints.

Why native cannot come from the browser

tile='auto' is the default and TILE_THRESHOLD is 1024, so every image
large enough to want a native export is tiled, and the page holds only a
downsampled overview. savefig re-encodes the backend at full resolution into
the snapshot; the menu disables that entry and says why. The test uses a
one-pixel stripe pattern the averaging overview renders flat — a ramp does not
discriminate, since at 1200 px it has only 256 grey levels.

Rasterising in Python was rejected once decorations became a requirement: the
LUT, gutter geometry and label engine would all have to be reimplemented, and
would drift.

Bugs fixed

• Key handlers ignored modifiers: Ctrl+C toggled the colourbar, Cmd+S
  flipped the colour scale to symlog.
• exportPNG mixed CSS-scaled rects with an unscaled extent, so a figure shrunk
  to fit a narrow cell composited into the top-left corner.
• Browser view state was never read back into Python, so save_html,
  to_html and figure_state silently reset the reader's zoom and pan.

Notes

47 new Playwright tests, no new baselines — exact sizes, known-LUT probes and
the literal _makeTheme constants. No gallery example: a menu and a keystroke
do not show in a static thumbnail. At native resolution decorations keep their
normal point sizes, so an 11 px tick label is small against a 4096 px image;
decoration_scale is the obvious follow-up.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.94161% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.86%. Comparing base (c1e8c36) to head (c682ca5).

Files with missing lines	Patch %	Lines
anyplotlib/_export.py	82.75%	20 Missing ⚠️
anyplotlib/figure/_figure.py	89.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
+ Coverage   90.85%   90.86%   +0.01%     
==========================================
  Files          40       41       +1     
  Lines        4603     4709     +106     
==========================================
+ Hits         4182     4279      +97     
- Misses        421      430       +9     

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