bug: M3 Dropdown on_select not fired on rapid consecutive selections (Web/CanvasKit)

[shi00]

Duplicate Check

• I have searched the opened issues and there are no duplicates

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Describe the bug

ft.Dropdown (Material 3) on_select event is not fired when a menu option is selected immediately after a previous selection on the same control, in the Web/CanvasKit runtime. The first selection fires on_select normally, but a rapid second selection is silently dropped. Waiting a few seconds before the next selection makes on_select fire again. ft.DropdownM2 (on_change-based) does not exhibit this behavior and fires on every selection.

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Code sample

Code

```python """Minimal reproduction: M3 ``ft.Dropdown`` ``on_select`` not fired on rapid consecutive selections.

How to reproduce (Web / CanvasKit, e.g. Chrome):
1. Run: python m3_dropdown_on_select_repro.py
2. In the opened browser page, click the dropdown and select "ERROR".
-> "on_select: ERROR" is appended to the event log.
3. Immediately click the dropdown again and select "DEBUG".
-> Expected: "on_select: DEBUG" | Actual: it is MISSING (the log still shows only ERROR).
4. Wait ~5s, then click the dropdown again and select "INFO".
-> "on_select: INFO" IS appended (menu state settled).

The same rapid second selection works fine with ft.DropdownM2 (on_change-based),
so this is specific to the Material 3 DropdownMenu event delivery in Web/CanvasKit.
"""

import logging

import flet as ft

logging.basicConfig(level=logging.INFO)

def main(page: ft.Page) -> None:
page.title = "M3 Dropdown on_select repro"
page.horizontal_alignment = ft.CrossAxisAlignment.CENTER

event_log = ft.Column()

def log_evt(msg: str) -> None:
    logging.info(msg)
    event_log.controls.append(ft.Text(msg))
    page.update()

def on_select(e: ft.ControlEvent) -> None:
    log_evt(f"on_select: {getattr(e.control, 'value', None)}")

def on_change(e: ft.ControlEvent) -> None:
    log_evt(f"on_change: {getattr(e.control, 'value', None)}")

dropdown_m3 = ft.Dropdown(
    label="Log level (M3, on_select)",
    options=[
        ft.dropdown.Option("ERROR", "错误 (ERROR)"),
        ft.dropdown.Option("DEBUG", "调试 (DEBUG)"),
        ft.dropdown.Option("INFO", "报告 (INFO)"),
    ],
    on_select=on_select,  # M3: on_select never fires for the 2nd rapid selection
)

dropdown_m2 = ft.DropdownM2(
    label="Log level (M2, on_change)",
    options=[
        ft.dropdown.Option("ERROR", "错误 (ERROR)"),
        ft.dropdown.Option("DEBUG", "调试 (DEBUG)"),
        ft.dropdown.Option("INFO", "报告 (INFO)"),
    ],
    on_change=on_change,  # M2: on_change fires reliably on every selection
)

page.add(
    ft.Text("Reproduce: select ERROR, then immediately select DEBUG in the same M3 dropdown."),
    dropdown_m3,
    dropdown_m2,
    ft.Divider(),
    ft.Text("Event log:"),
    event_log,
)

ft.app(main)

</details>

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**To reproduce**

1. Run the repro code: `python m3_dropdown_on_select_repro.py`
2. The app opens in the default browser (Web/CanvasKit).
3. Click the M3 dropdown ("Log level (M3, on_select)") and select "ERROR".
   - The event log shows `on_select: ERROR`.
4. Immediately click the same dropdown again and select "DEBUG".
   - Expected: `on_select: DEBUG` is appended.
   - Actual: nothing is appended (the second `on_select` is dropped).
5. Wait ~5 seconds, then click the same dropdown and select "INFO".
   - `on_select: INFO` IS appended.

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**Expected behavior**

`on_select` should fire for every user selection, including rapid consecutive selections on the same dropdown.

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**Screenshots / Videos**

<details open>
<summary>Captures</summary>
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</details>

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**Operating System**

Windows

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**Operating system details**

Windows Server 2022 (GitHub Actions windows-latest); reproducible in Chrome latest.

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**Flet version**

0.86.3 (flet-desktop 0.86.3)

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**Regression**

No, it isn't

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**Suggestions**

The M3 `DropdownMenu`/`MenuAnchor` event delivery in Web/CanvasKit appears to drop the selection event when the menu is reopened before the previous menu's teardown/animation completes. Please ensure the menu fully closes before the next selection event is accepted, or fire `on_select` based on the committed value change.

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**Logs**

<details open><summary>Logs</summary>
```console
[Paste your logs here]

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Additional details

Also reproduced in a real application: an E2E test that selects option A then immediately option B on the same ft.Dropdown intermittently never receives the second on_select event (the corresponding UI_ACTION log for the second selection is missing). ft.DropdownM2 with on_change works reliably in the same scenario.