[visibility_detector] add custom visible bounds

[lllttt06]

Description

Problem

Even though Stack or something is used and visibility_detector is covered with other widgets, VisibilityInfo.visibleFraction doesn't consider this situation.

Not only Stack, but something like ↓, visibility_detector doesn't consider BottomNavigationBar and AppBar.

Scaffold(
      appBar: // AppBar,
      extendBody: true,
      body: VisibilityDetector(),
      bottomNavigationBar: // BottomNavigationBar    
);

Solution

Added customVisibleBounds which is visibility_detector's detection area.

Related Issues

• [feature_request] Visibility detector margin support #417
• VisibilityDetector does not consider other widgets in a Stack #440

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[jamesderlin]

Hi, sorry I didn't notice this earlier. If you still want to merge this, you should:

1. Add some unit tests.
2. Add more documentation about what customVisibleBounds is and how it's supposed to be used? (For example, it'd be helpful to explain that it can specify a smaller area for the VisibilityDetector when some other rectangular widget overlaps it.) What coordinate system is it in?