Make sheet size independent of its child size

[fujidaiti]

The current sheet sizes itself to fit its child widget. This is fine in many cases, but making the sheet widget's size independent of the child widget's size allows the sheet to:

• Remove unnecessary bottom areas of the child widget, for example, the blank bottom area of the Navigator in a NavigationSheet, and
• Stretch the sheet height when the user over-drags it, creating the same visual effect as the iOS native bottom sheet as mentioned in Support iOS native modal sheet stretching behavior #169.

Implementation

Here's a rough sketch of the widget hierarchy:

SheetViewport
|- Sheet
   |- Material
      |- SheetContentViewport
         |- content-wrapper-widget (optional)
            |- ...
               |- SheetContentBoundary
                  |- content-widget

where,

• content-widget, which is the content of the sheet containing meaningful components for the user such as buttons, text, a list-view, etc.
• SheetContentBoundary, which notifies the SheetContentViewport of the content widget's size.
• content-wrapper-widget, a widget that is optionally inserted between the SheetContentBoundary and the SheetContentViewport. For example, Navigator serves as the content-wrapper-widget for the NavigationSheet. This widget may be larger than the SheetContentViewport and clipped by the Material in such cases.
• SheetContentViewport: a special OverflowBox that sizes itself to fit the content widget, and constraints the content wrapper widget to the size of the parent SheetViewport. This ensures that the content wrapper widget can be as large as the SheetViewport at most, while forcing the Material's size to match the content widget's size.
• Material, which draws the background color, the shadow, and the rounded corners for the sheet.