Phase 2 — extended form layout (0.9.0+)

This note complements PHASE2_PORTALS_AND_PREFETCH.md for layout-first Pydantic forms.

Field types in streamtree.forms_layout

Pydantic annotation (concept)	Widget	Binding helper
str / str \| None	TextInput	bind_str_fields
int / float / optional numeric	NumberInput	bind_numeric_fields
bool / bool \| None	Checkbox	bind_bool_fields

model_field_grid lays out named fields in rows of Columns; build_model_from_bindings merges str, numeric, and bool binding maps into a single model_validate call.

st.form and batch submit

Streamlit runs the full script on each rerun. A Form element in StreamTree maps to st.form; widgets inside read StateVar / FormState on each rerun after submit.

Batch validation: call build_model_from_bindings (and friends) inside the submit handler (e.g. a Button on_click that reads current StateVar values), not inside the worker thread. Surface failures with format_validation_errors next to the form.

Deferred (Phase 3)

Select boxes from Enums, date / datetime, file fields, and nested models are intentionally thin or out of scope here; add them when data-plane widgets and pins are defined.