Selection is sometimes not merged correctly

[psmyrek]

📝 Provide detailed reproduction steps (if any)

Similar case originally reported in https://github.com/cksource/ckeditor5-internal/pull/1006#issuecomment-931116656.

The mergeIntersectingRanges() function in packages/ckeditor5-engine/src/model/utils/selection-post-fixer.js does not work as expected in some cases using Firefox.

Let's assume the following:

1. We have 3 ranges to merge in the following order: A, B and C.

2. The A range is not intersecting with the B range.

3. The C range is intersecting with A and B.

     └── A ──┘  └── B ──┘
   └───────── C ──────────┘
   

4. The mergeIntersectingRanges() function returns incorrectly 2 ranges: A and B + C, because it checks only the previous range and not all previously handled ranges, as it should.

✔️ Expected result

The expected result is one merged range A + B + C.

❌ Actual result

Currently, this function returns 2 ranges: A and B + C, but the A range is intersecting with B + C (because C is intersecting with A as I described in the preconditions).

❓ Possible solution

This function should check all already handled ranges and not only the previous one.

📃 Other details

• Browser: Firefox

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