Draft: Remove special treatment of full sub-selection of a text field

Ensure consistent treatment of full sub-selection on font-family changes and on line-height changes by making full sub-selection same as any other (partial) selection. This fixes #1983.

The special treatment of full sub-selection was introduced in !796 (merged) as case 4 in the description. While it makes sense from the line-height point of view, this causes problems when combined with font-family changes.