Text editor: redo brings back stale edits, undo stops at nine, backspace breaks the cursor and undo loses it

Context

Welcome to the notes app team. The editor in editor/ implements all three levels described in README.md: typing and the cursor, copy and paste, and undo and redo. Since the last release, four reports have come in.

Fix as much as you can in 35 minutes. The checks are graded one by one, so each report you fix counts even if the rest is not done.

EDIT-11: Redo brings back edits from before a new edit

Report

Typed a, typed b, pressed undo, typed c. The text was ac. Pressing redo turned it into ab, losing the c.

Acceptance criteria

• After a new edit, redo does nothing until something is undone again.

EDIT-12: Undo stops one edit early

Report

The release notes promise the last 10 edits can be undone. After typing 10 single letters, 10 undos leave the first letter behind.

Acceptance criteria

• The last 10 edits can be undone; only an 11th pushes the oldest out.

EDIT-13: Backspacing past the start breaks the cursor

Report

With abc typed and the cursor at the end, delete(10) empties the document, but get_cursor() then reports -7 and the next characters typed land in the wrong place.

Acceptance criteria

• delete(count) with more than the characters before the cursor removes those characters and leaves the cursor at 0.

EDIT-14: Undo does not put the cursor back

Report

Typed abc, moved the cursor to 1, typed X, pressed undo. The text is abc again, but the cursor stays at 2, so the next key lands after the b.

Acceptance criteria

• After undo, the cursor is exactly where it was just before the undone edit.

Notes

• Run pytest tests/visible. Three tests fail on the current code; not every report has a failing test yet.
• Practice task modelled on candidate reports of text editor undo and redo rounds at Microsoft and Amazon. Original StackYard code, not the actual question; not affiliated with Microsoft or Amazon.