Fuzzier Mark Generation

When creating Mark objects (to highlight selected text for a given source) we rely on strict fragment selector indexes or exact text match. There are plenty of scenarios that will screw up this UI, such as if the source text itself changes. Improve the Marking system to accommodate small inaccuracies or content fluctuation.

Start by locating the Mark objects and Marking system that create highlights from fragment-selector indexes or exact text matches. Read how those inputs are currently resolved, then define behavior for small inaccuracies and source-text changes; done means marks continue to highlight the intended selection when content fluctuates.

Written by the indexing model from the issue text.