Refactor: extract SelectionModel, UndoStack, Clipboard from TextBox (…

[asshifasultana]

Summary

This PR is Phase 3 of an ongoing God Class decomposition refactor for TextBox.

It extracts three independent concerns from TextBox.java (≈950 lines) into focused collaborator classes under the gui2.textbox package, improving separation of concerns and maintainability.

No behavior changes are introduced.

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Problem

TextBox previously handled multiple unrelated responsibilities in a single class, including:

• Text content management

• Line storage and updates

• Caret positioning

• Selection state

• Undo/redo history

• Clipboard buffer

• Rendering logic

• Change listeners

This made the class difficult to maintain, test, and extend.

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Solution

Extracted the following responsibilities into dedicated helper classes:

New Class | Responsibility | Lines -- | -- | -- TextBoxSelectionModel | Manages selection start/end and text extraction | 47 TextBoxUndoStack | Maintains bounded LIFO history (size 50) for undo/redo | 60 TextBoxClipboard | Handles clipboard read/write for copy/paste | 41

TextBox now delegates these responsibilities through thin wrapper methods, preserving the existing public API and behavior.

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Verification

• mvn clean compile → BUILD SUCCESS

• All TextBox-related tests pass

• Existing unrelated failures in Button / CheckBox remain unchanged

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Files Changed

4 files changed, +215 / -0

• gui2/TextBox.java (+67) — Added delegator methods

• gui2/textbox/TextBoxClipboard.java (+41) — New

• gui2/textbox/TextBoxSelectionModel.java (+47) — New

• gui2/textbox/TextBoxUndoStack.java (+60) — New