Explain figure rendering failures caused by kernel subshells

docs/getting-started/installation.md

@@ -19,3 +19,22 @@ conda install -c conda-forge plopp
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+
+## Interactive figures in JupyterLab
+
+Interactive figures (`%matplotlib widget`) can fail to render in JupyterLab 4.4 and later,
+with math text parse errors, blank figures, or kernel crashes.
+JupyterLab routes widget messages over kernel subshells, which ipykernel 7 and later
+service on their own threads.
+Drawing a live canvas then races with figure creation during cell execution,
+and Matplotlib is not thread-safe
+(see [ipympl#610](https://github.com/matplotlib/ipympl/issues/610)).
+
+A lock added in ipympl removes the math text parse errors, but the blank figures and
+crashes remain, since the canvas frame and resize handlers still run concurrently with
+drawing.
+Until that is addressed, either
+
+- set `commsOverSubshells` to `disabled` in the JupyterLab settings editor and restart
+  JupyterLab (the setting only applies to newly connected kernels), or
+- install `ipykernel<7`.