Make limiter element more configurable

[oskarwrobel]

Let's assume that someone has a sticky header over the editable, and doesn't want to show BPV over the header and doesn't want to hide BPV under the header.

There is no way to add sticky header height to the limiter top offset.

[fredck]

What if [limiter top-offset] == [skicky header top-offset]? I mean, the header is floating above the limiter?

For sure, having a limiter element is important as it covers many scenarios. Still, we may need to eventually provide configurable padding somehow, in the [ top, right, bottom, left ] format.

[oleq]

Since https://github.com/ckeditor/ckeditor5-utils/issues/157, you can define target as Functions. ContextualToolbar uses this approach.

All we need is to enable this thing for limiter. It's a 1LOC enhancement (+docs). Then, such function limiter function would be totally configurable and as specific as the implementation requires. For the case described by @oskarwrobel:

limiter: () => {
    const limiterRect = new Rect( limiterElementInDOM );
    const stickyHeaderRect = new Rect( stickyHeaderInDOM );

    // Adjust the limiterRect to consider the stickyHeader. We could even 
    // introduce some sugar like Rect#substract() similar to existing Rect#getIntersection()
    limiterRect.top += stickyHeaderRect.height;
    limiterRect.height -= stickyHeaderRect.height;
    // etc.

    return limtierRect;
}

Still, we may need to eventually provide configurable padding somehow, in the [ top, right, bottom, left ] format.

It would fail to meet more complex scenario requirements very soon. Then we'd be back exactly where we started. With the functional limiter, you can return any sort of Rect, e.g. in this case

  +--------------------------------------------+
  |                                            |
  |           The limiter element              |
  |                                            |
  |                                            |
  |     +--------------------------------+     |
  |     |                                |     |
  |     |     Some "taboo" zone          |     |
  |     |                                |     |
  |     +--------------------------------+     |
  |                                            |
  |                                            |
  |                                            |
  |                                            |
  +--------------------------------------------+

paddings would do no good but it's trivial case for Rect manipulation.

[oskarwrobel]

We need to:

• make possible to use function as limiter
• make possible to override default limiter for current balloons (Link, ContextualToolbar).