Set a controlled widget back after an event that caused a render

[maartenbreddels]

Why

Take a controlled widget, where the element gives the value and a handler maps the new value to state:

value, set_value = use_state("AA")
def on_value(v):
    set_value(v.upper())
    set_count(lambda n: n + 1)  # some other state, e.g. in a sibling component
w.Text(value=value, on_value=on_value)

Type "Bb": the handler sets "BB", which is the state we already have, and it changes the count, so a render happens.

• The default renderer walks the whole tree in that render and sets every widget back to its element's values. The text shows "BB".
• The fast renderer (REACTON_FAST=1) skips the component that did not change, so the text keeps showing "Bb" while the state is "BB".

This is a case of vuejs/vue#13237.

handler	default, master	fast, master	this PR, both
a sibling's state changes too (render)	"BB"	"Bb" ✗	"BB"
no other state change (no render)	"Bb"	"Bb"	"Bb" (unchanged)

What

The event listener is now a small class, _TraitListener, that remembers the element that currently owns the widget. That element is updated when a new element keeps the same callback. After a handler that caused a render, the listener sets the trait back to that element's value. So the fast renderer now gives the default renderer's result.

• If the render replaced the listener (new callback), it does nothing: the new element has already been applied.
• If the element gives no value (uncontrolled), the widget keeps what the user entered.
• It also does nothing during a render, when the render closed the widget, or when the value holds elements.

When nothing rendered, nothing changes, in both renderers. A first version always set the widget back, like React and like #45. That broke 5 of solara's input_test.py tests: InputInt/InputFloat with continuous_update=False pass the old value, ignore v_model changes while you type, and take the new value on blur, so every keystroke snapped back. Fixing the no-render case needs a change in solara first.

Tests

In both renderers:

• test_controlled_widget_no_state_change_sibling_renders[layout] and test_controlled_widget_no_state_change_parent_renders: fail on master in the fast renderer, pass in the default renderer.
• test_controlled_widget_same_callback_new_value: the callback is the same object on every render, so the value set back must come from the new element.
• test_controlled_widget_no_render_keeps_user_value: pins the solara continuous_update=False behavior.
• test_uncontrolled_widget_keeps_its_value, test_controlled_widget_removed_by_handler: guard tests.
• Solara's unit suite: the same results as master, with REACTON_FAST=0 and =1.

🤖 Generated with Claude Code

[maartenbreddels]

not worth a performance hit maybe?