[Fix] st.selectbox set via session state does not restore initial value
#13438
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st.selectbox set via session state does not restore initial value
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Pull request overview
This PR fixes a bug where st.selectbox would incorrectly revert to displaying the first option in the UI when its value was set via session state and the user opened/closed the dropdown without making a selection. The fix ensures the valueBeforeRemoval ref is reset when the value changes externally, preventing it from restoring a stale value on blur.
Key changes:
- Modified the
useExecuteWhenChangedhook callback in Selectbox to resetvalueBeforeRemoval.currenttonullwhenpropValuechanges - Added comprehensive test coverage with both frontend unit tests and E2E tests
Reviewed changes
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Show a summary per file
| File | Description |
|---|---|
frontend/lib/src/components/shared/Dropdown/Selectbox.tsx |
Resets valueBeforeRemoval ref to null when propValue changes externally to prevent stale value restoration |
frontend/lib/src/components/shared/Dropdown/Selectbox.test.tsx |
Adds unit test verifying selectbox preserves value after prop change and blur without selection |
e2e_playwright/st_selectbox.py |
Adds test app section with buttons to trigger session state changes for regression testing |
e2e_playwright/st_selectbox_test.py |
Adds E2E test verifying UI stays in sync when value set via session state and dropdown opened/closed |
e2e_playwright/__snapshots__/linux/st_selectbox_test/st_selectbox-dynamic_*.png |
Updated snapshots for dynamic selectbox tests to reflect the additional test content |
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📉 Frontend coverage change detectedThe frontend unit test (vitest) coverage has decreased by 0.0000%
✅ Coverage change is within normal range. |
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lukasmasuch
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Dec 23, 2025
e2e_playwright/st_selectbox.py
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| # Regression test for https://github.com/streamlit/streamlit/issues/13435 | ||
| # Test that selectbox UI stays in sync when value is set via session_state | ||
| # and user opens/closes dropdown without selecting | ||
| with st.container(horizontal=True): |
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It might be better to put this under the dynamic selectbox to prevent subpixel flakiness
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Describe your changes
When you set the
st.selectboxvalue via session state, and then open/close the selectbox dropdown menu without making a selection, thest.selectboxvalue in the UI transitions to the first option (even though session state value is correct).Typically, our
valueBeforeRemovalref handles when user starts typing to filter options - it stores the current value so it can be restored if they blur without selecting. This is initialized on mount with the initial selectbox value and only resets tonullwhen the user makes a selection viahandleChange. It is not updated whenpropValuechanges externally via session state. So this PR addresses the issue by resetting the ref tonullwhenpropValuechanges, which ensures the stale ref doesn't interfere after external value changes.GitHub Issue Link (if applicable)
Closes #13435
Testing Plan