[material_ui] Stop MenuAnchor from covering its anchor

[RootHex200]

When a menu opens upwards and it is too tall to fit above its button, the menu is pushed down to the bottom edge of the screen and covers the button completely. You cannot see the button you just tapped.

The reason is that MenuAnchor only limited the menu height to the size of the whole screen. But a menu can only use the space above the button or the space below it, never both. So when the menu did not fit above, the positioning code ran out of options and pinned it to the bottom edge, right on top of the button.

Now the menu height is limited to the bigger of those two spaces. A menu that is taller than that becomes shorter and scrolls, instead of growing over the button.

Addresses flutter/flutter#161474

before:

after:

I added a test in test/menu_anchor_test.dart that puts a menu at the bottom of a short screen and checks that the bottom of the menu never goes past the top of the button.

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[gemini-code-assist]

Code Review

This pull request modifies the getConstraintsForChild method in _MenuLayout to cap the height of vertical menus to the available space above or below the anchor, preventing the menu from covering the anchor when it is too tall. It also adds a corresponding regression test and a changelog entry. There are no review comments, and I have no feedback to provide.