Bug 491412

Currently, when an end of block is detected, it triggers a line-by-line readback until an opening is found. As the character or word searched for can be found in comments and strings, there's a syntax parsing logic syntax that makes the whole thing rather complex. As the API for attributes is also very limited, you can't rely on it without going character by character...

However, the syntax part is already handled by SyntaxHighlighting, which also knows the beginning and end of the region.

With an API that allows you to navigate and retrieve region positions, indentation can be simplified to something like:

```js
// a syntax in closing blocks are 'end'
if (document.defStyleNum(line, curColumn) === dsControlFlow) {
  const kw = document.wordAt(line, curColumn);
  if (kw === "end") {
    const column = document.firstColumn(line);
    // 'end' is the first word of the line
    if (column > 0 && curColumn === column + kw.length) {
      const cur = document.matchingOpeningRegion(line, column); // new API
      // beginRegion found
      if (cur) {
        return document.firstVirtualColumn(cur.line);
      }
    }
  }
}
```

As regions can overlap, APIs would be needed to retrieve several of them and search, for example, for the previous unclosed opening (as in the case of line indentations in a hash and array). Also functions to convert a region name into an id that can then be used for the search (faster than systematically passing a string). But we'd need to introduce a function called automatically when the syntax changes.

Search functions would also be handy to use at attribute level (as mentioned above, they are very limited). The only search functions that make use of them are `document.find` / `document.rfind`, but are in fact 

As regions can overlap, we also need APIs that return several of them, and search for the previous unclosed opening, for example (as in the case of line indentations in a hash and array). Also functions to convert a region name into an id that can then be used for the search (faster than systematically passing a string). But we'd need to introduce a function called automatically when the syntax changes.

Search functions would also be handy to use at attribute level (as mentioned above, they are very limited). The only search functions that make use of them are `document.find` / `document.rfind`, but are actually text-centric.