Plan:

Step 1:

Type text
Render text
Store text in simples form (char[])

Step 2:

Caret movement, left/right/backspace/delete/insert
Implement interface for data structure and basic text operations

Step 3:

Detect line feed special character and render multiple lines of text
Caret movement up/down

Step 4:

[Maybe] Update our data structure

Step 5:

Open and read file
Save file on disk

Step 6:

Viewport and scrolling
Scrollbar/ file minimap

Step 7:

Document markup model (simplest impl: sorted list of highlighters)
Update rendering

Step 8:

Text selection
Copy/Cut/Paste support

Step 9:

Language services
Lexical analyser and lexical syntax highlighter

Step 10:

Project model and [maybe] multiple text editor tabs
Simple tree-like project view using default controls from library

Step 11:

Introduce background processing and move lexical analysis and project mode processing to background
Some way to manage concurrency

Step 12:

Syntax analyser
Show syntax errors

Step 13:

VFS and Caches

Step 14:

Build symbol tables and perform semantic analysis (related to searching, auto-completion, go-to-declaration, etc.)
Type inference and type checking
Show semantic errors

Step 15:

Simple auto-completion
Simple "Go-to-definition"

Step 16:

Code modification and transactions
Some refactoring

Step 17:

Refine, optimize, rewrite basic data structures if needed