Ensure Date(Time)Picker does not clear on partial entry

src/panel_material_ui/widgets/DateTimePicker.jsx

@@ -120,15 +120,23 @@ export function render({model, view, el}) {
 
   // Safely update the model value
   function updateModelValue(date) {
-    if (!date || !date.isValid()) {
+    if (!date) {
       if (clearable) {
         lastCommittedRef.current = null;
         setValue(null);
+        setInternalValue(null);
         model.value = null;
       }
       return;
     }
 
+    if (!date.isValid()) {
+      // Partial/incomplete edit (e.g. only some date sections filled in) -
+      // do not clear the committed value, just restore the display.
+      setInternalValue(value)
+      return;
+    }
+
     const formattedDate = formatDateForPython(date);
     // Skip update if this is the same value we just committed
     const currentTimestamp = date.valueOf();
@@ -151,7 +159,14 @@ export function render({model, view, el}) {
 
   // Handle changes from the date picker UI
   const handleChange = (newValue) => {
-    setInternalValue(newValue)
+    // Ignore transient invalid states produced while a section is being
+    // typed (e.g. only the year digit typed so far) so the other, already
+    // filled-in sections aren't wiped from the display mid-edit. A fully
+    // empty field is reported as `null`, not an invalid date, so that case
+    // is still tracked to support clearing.
+    if (!newValue || newValue.isValid()) {
+      setInternalValue(newValue)
+    }
 
     // For direct calendar selection, update immediately
     if (isCalendarOpen && newValue && newValue.isValid()) {

tests/ui/widgets/test_date_picker.py

@@ -92,3 +92,40 @@ def test_datepicker_clearable_manual_delete_propagates_none(page):
     input_el.press("Tab")
 
     wait_until(lambda: widget.value is None, page)
+
+
+def test_datepicker_clearable_partial_edit_does_not_clear(page):
+    # Regression test: editing a single date section (e.g. typing into the
+    # year) while other sections are still unset must not wipe the whole
+    # value, even when clearable=True.
+    widget = DatePicker(value=dt.date(2026, 8, 13), clearable=True)
+    serve_component(page, widget)
+
+    input_el = page.locator(".MuiPickersInputBase-input")
+    wait_until(lambda: "2026-08-13" in input_el.input_value(), page)
+
+    year_section = page.locator("span[aria-label='Year']")
+    year_section.click()
+    page.keyboard.press("2")
+
+    assert widget.value == dt.date(2026, 8, 13)
+    assert "08-13" in input_el.input_value()
+
+    page.keyboard.press("Tab")
+
+    assert widget.value == dt.date(2026, 8, 13)
+
+
+def test_datepicker_edit_single_section_commits(page):
+    widget = DatePicker(value=dt.date(2026, 8, 13), clearable=True)
+    serve_component(page, widget)
+
+    input_el = page.locator(".MuiPickersInputBase-input")
+    wait_until(lambda: "2026-08-13" in input_el.input_value(), page)
+
+    year_section = page.locator("span[aria-label='Year']")
+    year_section.click()
+    page.keyboard.type("2030")
+    input_el.press("Tab")
+
+    wait_until(lambda: widget.value == dt.date(2030, 8, 13), page)