Section on Text Authoring/Editing UX

[behnam]

As a follow up to the discussion on #113, we want to consider to add a section, possibly as an Appendix, on layout-related matters in user interfaces. Example questions to answer would be:

• How text highlighting should work. Specifically, most UI code need to know word boundaries, line boundaries, paragraph boundaries.

• Cursor positioning and cursor movement. Specifically, cursor movement in 1) RTL paragraph vs LTR paragraph, 2) inside a bidi level run with different direction of the paragraph, and 3) on the boundary of bidi level runs.

• Cursor shape to denote paragraph direction and/or direction at the current position.

• Mechanical paragraph direction setting as Higher-Level Protocols. Specifically, whether LRM/RLM/ALM markers should be inserted if an L/R/AL letter is inserted and deleted on an empty paragraph.

• Behavior of inserting/replacing LTR/RTL character in LTR/RTL context. Specifically, if there should be any mechanical marker-insertion or mechanical embedding/isolation.

• Behavior of copy-n-pasting LTR/RTL text in LTR/RTL context. Specifically, if there should be any mechanical marker-insertion or mechanical embedding/isolation.