notify::selected passes a GParamSpec* ahead of the user data and
GtkSpinButton::value-changed does not, so on_math_param_changed read the
pspec as its MathDialog* and faulted on the first field it touched --
which is to say on opening the math editor and picking a source.
The probe missed it because filling the form sets the dropdown to the
value it already holds, and GObject emits no notify for that: a channel
sourcing channels().front() picks index 0 on a dropdown already at 0.
math_probe now opens on a channel sourcing the *last* acquired signal,
which does move the selection, and reads back the source the handler
wrote into the node.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>