Hacky workaround to fix some stuff with InputField

ScriptEditor.cs

using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;
using System;
using System.Collections;
using System.IO;

public class ScriptEditor : InputField {
    public Text lineNumbers;

    private RectTransform rectTransform;

    protected override void Start() {
        rectTransform = GetComponent<RectTransform>();

        onValueChange.AddListener(new UnityEngine.Events.UnityAction<string>(TextChanged));
        onValueChange.AddListener(new UnityEngine.Events.UnityAction<string>(ResizeInput));
        onValueChange.AddListener(new UnityEngine.Events.UnityAction<string>(UpdateLineNumbers));
        onValidateInput = AutoIndent;
    }

    // Fix InputField not setting Text component's text
    private void TextChanged(string iText) {
        m_Text = iText;
        UpdateLabel();
        m_TextComponent.text = iText;
    }

    // Resize input field as new lines get added
    private void ResizeInput(string iText) {
        string fullText = text;

        Vector2 extents = textComponent.rectTransform.rect.size;
        var settings = textComponent.GetGenerationSettings(extents);
        settings.generateOutOfBounds = false;
        var prefHeight = new TextGenerator().GetPreferredHeight(fullText, settings) + 16;

        if(prefHeight > textComponent.rectTransform.rect.height - 16 || prefHeight < textComponent.rectTransform.rect.height + 16) {
            rectTransform.sizeDelta = new Vector2(rectTransform.sizeDelta.x, prefHeight);
        }
    }

    // Update the line numbers
    private void UpdateLineNumbers(string text) {
        int numLines = 0;
        foreach(char c in text) {
            if(c == '\n') numLines++;
        }
        
        string lineNumText = "";
        for(int i = 0; i <= numLines; i++) {
            lineNumText += (i + 1) + "\n";
        }
        lineNumbers.text = lineNumText;
    }

    // Automatic indentation matching
    private char AutoIndent(string input, int charIndex, char addedChar) {
        if(addedChar == '\n') {
            string prevLine;
            int prevLineStart = input.LastIndexOf('\n', Mathf.Max(charIndex - 1, 0)) + 1;

            prevLine = input.Substring(prevLineStart, charIndex - prevLineStart);

            int numWhitespace = 0;
            string autoWhitespace = "";
            while(numWhitespace < prevLine.Length && (prevLine[numWhitespace] == '\t' || prevLine[numWhitespace] == ' ')) {
                autoWhitespace += prevLine[numWhitespace];
                numWhitespace++;
            }

            text = input.Insert(charIndex, '\n' + autoWhitespace);
            caretPosition = charIndex + 1 + numWhitespace;

            return '\0';
        }

        return addedChar;
    }

    //Returns string on given line number
    public string GetTextOnLineNumber( int lineNumber ) {
        using ( StringReader reader = new StringReader( text ) ) {
            int currLine = 0;
            string line;
            while ( (line = reader.ReadLine()) != null ) {
                if ( currLine == lineNumber ) {
                    return line;
                } else {
                    currLine++;
                }
            }
            return null;
        }
    }


    // Overriding InputField
    // InputField source: https://bitbucket.org/Unity-Technologies/ui/src/7c219d27562749b5581b017103c6947cebca6536/UnityEngine.UI/UI/Core/InputField.cs

    // this code, is bananas. B.A.N.A.N.A.S.
    public void SetText(string value) {
        // Try
        try {
            m_Text = value;
            UpdateLabel();

            if(onValueChange != null)
                onValueChange.Invoke(m_Text);
        } catch {
            SetTextForReal(value);
        }
    }
    private void SetTextForReal(string value) {
        // Try harder
        m_Text = value;
        UpdateLabel();

        if(onValueChange != null)
            onValueChange.Invoke(m_Text);
    }

    private int GetUnclampedCharacterLineFromPosition(Vector2 pos, TextGenerator generator)
    {
        if (!multiLine)
            return 0;

        float height = m_TextComponent.rectTransform.rect.yMax;

        // Position is above first line.
        if (pos.y > height)
            return -1;

        for (int i = 0; i < generator.lineCount; ++i)
        {
            float lineHeight = generator.lines[i].height / m_TextComponent.pixelsPerUnit;
            if (pos.y <= height && pos.y > (height - lineHeight))
                return i;
            height -= lineHeight;
        }

        // Position is after last line.
        return generator.lineCount;
    }

    private static int GetLineEndPosition(TextGenerator gen, int line)
    {
        line = Mathf.Max(line, 0);
        if (line + 1 < gen.lines.Count)
            return gen.lines[line + 1].startCharIdx - 1;
        return gen.characterCountVisible;
    }

    protected new int GetCharacterIndexFromPosition(Vector2 pos)
    {
        TextGenerator gen = m_TextComponent.cachedTextGenerator;

        if (gen.lineCount == 0)
            return 0;

        int line = GetUnclampedCharacterLineFromPosition(pos, gen);
        if (line < 0)
            return 0;
        if (line >= gen.lineCount)
            return gen.characterCountVisible;

        int startCharIndex = gen.lines[line].startCharIdx;
        int endCharIndex = GetLineEndPosition(gen, line);

        for (int i = startCharIndex; i < endCharIndex; i++)
        {
            if (i >= gen.characterCountVisible)
                break;

            UICharInfo charInfo = gen.characters[i];
            Vector2 charPos = charInfo.cursorPos / m_TextComponent.pixelsPerUnit;

            float distToCharStart = pos.x - charPos.x;
            float distToCharEnd = charPos.x + (charInfo.charWidth / m_TextComponent.pixelsPerUnit) - pos.x;
            if (distToCharStart < distToCharEnd)
                return i;
        }

        return endCharIndex;
    }

    public override void OnPointerDown(PointerEventData eventData)
    {
        EventSystem.current.SetSelectedGameObject(gameObject, eventData);

        base.OnPointerDown(eventData);

        Vector2 pos = ScreenToLocal(eventData.position);
        caretPosition = GetCharacterIndexFromPosition(pos) + m_DrawStart;

        UpdateLabel();
        eventData.Use();
    }
}