# Filter Dropdown Click Fix

## Problem

dev-21.5.1 correctly rendered filter buttons on the first row of the selected table range, for example `商品 / 価格 / 数量 / 金額`.  However, clicking the small `▼` button could fail to show the dropdown because the grid has document-level transient overlay closing logic and the button was opened only from the delegated `click` event.

## Fix

- Open filter dropdowns from `mousedown` on `.ng-filter-button`.
- Stop event bubbling immediately so the document-level closer does not consume the gesture.
- Keep a defensive `click` handler only to suppress late cell-selection clicks.
- Anchor the popup to the button `getBoundingClientRect()` instead of raw click coordinates.
- Append the popup to `document.body` and clamp it within the viewport.
- Let mousedown/click inside `.ng-filter-menu` stop propagation so the menu stays open while the user interacts with inputs and checkboxes.

## Design Note

Filter dropdowns are spreadsheet editing UI, not Demo UI.  Therefore the fix is implemented in `native_grid_renderer` and `themes/base/grid_layout.css`.  Demo maximize and option panels remain separated in `demo/assets/`.