Any Python code that only mentioned "matplotlib" (a comment, a string, or an importlib.util.find_spec("matplotlib") check) failed with ModuleNotFoundError before a single line of it ran. The plt.show() patch was applied on a plain substring match, but matplotlib is only installed when the code actually imports it, so the patch's own import crashed the run. The user's try/except could not catch it, because their code never started. The same thing broke the code editor's Python formatter on any code that imports matplotlib, since that run only installs black.
The patch now also requires matplotlib to be present in the runtime's loaded packages, in both the worker and the sandboxed iframe host. Real matplotlib code still gets inline PNG output from plt.show(), and code that merely mentions matplotlib runs unchanged.
Gating on the callers' import regex instead was also tested and still fails the formatter case, because there the code containing the import sits inside a string while only black is installed.
Fixes#29894
The sandboxed Pyodide host (used when ENABLE_PYODIDE_FILE_PERSISTENCE is
disabled, the default) embeds its script in a String.raw template. The
matplotlib show() override was written with '\\t' escapes as if in a normal
string context, but String.raw preserves them verbatim, so the iframe's
script parser turns them into literal backslash-t characters in the
generated Python source. Pyodide then fails to compile any code that
triggers the matplotlib patch with "SyntaxError: unexpected character
after line continuation character", which is why matplotlib only worked
with the file persistence worker path enabled.
Use single '\t' escapes instead: String.raw keeps them as-is in the
script text and the sandbox's JS parser produces real tab indentation,
matching the working implementation in pyodide.worker.ts.
Fixes#26660
