Tried adding some sort of document filter.

src/main/java/nl/andrewlalis/blockbookbinder/BlockBookBinder.java

@@ -10,8 +10,8 @@ import javax.swing.*;
  */
 public class BlockBookBinder {
 	public static void main(String[] args) {
+		FlatDarkLaf.install();
 		SwingUtilities.invokeLater(() -> {
-			FlatDarkLaf.install();
 			var mainFrame = new MainFrame();
 			mainFrame.setupAndShow();
 		});

src/main/java/nl/andrewlalis/blockbookbinder/control/CleanSourceActionListener.java

@@ -1,45 +0,0 @@
-package nl.andrewlalis.blockbookbinder.control;
-
-import nl.andrewlalis.blockbookbinder.view.SourceTextPanel;
-
-import java.awt.event.ActionEvent;
-import java.awt.event.ActionListener;
-
-/**
- * Listener for an action where the source text is 'cleaned' by removing any
- * trailing whitespace, and removing unnecessary newlines.
- */
-public class CleanSourceActionListener implements ActionListener {
-	private final SourceTextPanel sourceTextPanel;
-
-	public CleanSourceActionListener(SourceTextPanel sourceTextPanel) {
-		this.sourceTextPanel = sourceTextPanel;
-	}
-
-	@Override
-	public void actionPerformed(ActionEvent e) {
-		final String source = this.sourceTextPanel.getSourceText();
-		String updated = source.trim()
-				.replaceAll("(?>\\v)+(\\v)", "\n\n") // Replace large chunks of newline with just two.
-				.replace("  ", " "); // Remove any double spaces.
-		updated = this.removeNewlineWrapping(updated);
-		this.sourceTextPanel.setSourceText(updated);
-	}
-
-	private String removeNewlineWrapping(String source) {
-		final StringBuilder sb = new StringBuilder(source.length());
-		final char[] sourceChars = source.toCharArray();
-		for (int i = 0; i < sourceChars.length; i++) {
-			char c = sourceChars[i];
-			if (
-					c == '\n'
-							&& (i - 1 >= 0 && !Character.isWhitespace(sourceChars[i - 1]))
-							&& (i + 1 < sourceChars.length && !Character.isWhitespace(sourceChars[i + 1]))
-			) {
-				c = ' ';
-			}
-			sb.append(c);
-		}
-		return sb.toString();
-	}
-}

src/main/java/nl/andrewlalis/blockbookbinder/control/ConvertToBookActionListener.java

@@ -1,29 +0,0 @@
-package nl.andrewlalis.blockbookbinder.control;
-
-import nl.andrewlalis.blockbookbinder.model.build.BookBuilder;
-import nl.andrewlalis.blockbookbinder.view.book.BookPreviewPanel;
-import nl.andrewlalis.blockbookbinder.view.SourceTextPanel;
-
-import java.awt.event.ActionEvent;
-import java.awt.event.ActionListener;
-
-/**
- * Action listener that, when activated, converts the text from the source panel
- * into a formatted book.
- */
-public class ConvertToBookActionListener implements ActionListener {
-	private final SourceTextPanel sourceTextPanel;
-	private final BookPreviewPanel bookPreviewPanel;
-
-	public ConvertToBookActionListener(SourceTextPanel sourceTextPanel, BookPreviewPanel bookPreviewPanel) {
-		this.sourceTextPanel = sourceTextPanel;
-		this.bookPreviewPanel = bookPreviewPanel;
-	}
-
-	@Override
-	public void actionPerformed(ActionEvent e) {
-		this.bookPreviewPanel.setBook(
-				new BookBuilder().build(this.sourceTextPanel.getSourceText())
-		);
-	}
-}

src/main/java/nl/andrewlalis/blockbookbinder/model/Book.java

@@ -22,6 +22,11 @@ public class Book {
 		this.pages.add(page);
 	}
 
+	public BookPage getPage(int index) {
+		if (index < 0 || index >= this.pages.size()) return null;
+		return this.pages.get(index);
+	}
+
 	/**
 	 * Gets a book containing the pages specified by the range.
 	 * @param firstIndex The index of the first page to include.

src/main/java/nl/andrewlalis/blockbookbinder/model/BookPage.java

@@ -20,6 +20,24 @@ public class BookPage {
 		return true;
 	}
 
+	public String getLine(int index) {
+		if (index < 0 || index >= this.lines.size()) {
+			return null;
+		}
+		return this.lines.get(index);
+	}
+
+	public int getLineIndexAtOffset(int offset) {
+		int lineIndex = 0;
+		String line = this.getLine(lineIndex);
+		if (line == null) return -1;
+		while (offset - line.length() > 0) {
+			offset-= line.length();
+			line = this.getLine(++lineIndex);
+		}
+		return lineIndex;
+	}
+
 	public boolean hasContent() {
 		return !this.lines.isEmpty();
 	}

src/main/java/nl/andrewlalis/blockbookbinder/model/build/BookBuilder.java

@@ -2,7 +2,7 @@ package nl.andrewlalis.blockbookbinder.model.build;
 
 import nl.andrewlalis.blockbookbinder.model.Book;
 import nl.andrewlalis.blockbookbinder.model.BookPage;
-import nl.andrewlalis.blockbookbinder.model.CharWidthMapper;
+import nl.andrewlalis.blockbookbinder.util.CharWidthMapper;
 import nl.andrewlalis.blockbookbinder.util.ApplicationProperties;
 
 import java.util.ArrayList;
@@ -16,7 +16,7 @@ public class BookBuilder {
 	 */
 	public Book build(String source) {
 		final int maxLines = ApplicationProperties.getIntProp("book.page_max_lines");
-		List<String> lines = this.convertSourceToLines(source);
+		List<String> lines = this.convertSourceToLines(this.cleanSource(source));
 		Book book = new Book();
 		BookPage page = new BookPage();
 		int currentPageLineCount = 0;
@@ -38,6 +38,21 @@ public class BookBuilder {
 		return book;
 	}
 
+	/**
+	 * Cleans a given source text, by removing superfluous newlines that won't
+	 * look too nice in minecraft text, and by erasing manually-imposed single
+	 * newlines that may be in the source text to format it in a normal editor.
+	 * @param source The source text.
+	 * @return The cleaned string.
+	 */
+	public String cleanSource(String source) {
+		return source.trim()
+			.replaceAll("(?>\\v)+(\\v)", "\n\n") // Replace large chunks of newline with just two.
+			.replaceAll("\\S\n\\S", " ") // Unwrap previously-imposed single-line wrapping.
+			.replaceAll("\t", "  ") // Replace tabs with single-spaces, due to space constraints.
+			.replaceAll(" [ ]+", " "); // Remove any superfluous spaces.
+	}
+
 	/**
 	 * Converts the given source string into a formatted list of lines that can
 	 * be copied to a minecraft book.

src/main/java/nl/andrewlalis/blockbookbinder/util/CharWidthMapper.java

@@ -1,11 +1,14 @@
-package nl.andrewlalis.blockbookbinder.model;
+package nl.andrewlalis.blockbookbinder.util;
 
 import lombok.Getter;
-import nl.andrewlalis.blockbookbinder.util.ApplicationProperties;
 
 import java.util.HashMap;
 import java.util.Map;
 
+/**
+ * This class provides the ability to determine the length of a string, in
+ * pixels, according to Minecraft's internal font.
+ */
 public class CharWidthMapper {
 	@Getter
 	private static final CharWidthMapper instance = new CharWidthMapper();
@@ -21,6 +24,16 @@ public class CharWidthMapper {
 		return this.charWidthMap.getOrDefault(c, 6);
 	}
 
+	public int getWidth(String s) {
+		if (s.length() == 0) return 0;
+		int width = getWidth(s.charAt(0));
+		for (int i = 1; i < s.length(); i++) {
+			final char c = s.charAt(i);
+			width += this.getWidth(c) + 1;
+		}
+		return width;
+	}
+
 	private void initCharWidthMap() {
 		this.charWidthMap.put(' ', 3);
 		this.charWidthMap.put('!', 1);
@@ -49,6 +62,7 @@ public class CharWidthMapper {
 		this.charWidthMap.put('|', 1);
 		this.charWidthMap.put('}', 3);
 		this.charWidthMap.put('~', 6);
+		this.charWidthMap.put('\n', 0);
 
 		final int defaultWidth = ApplicationProperties.getIntProp("book.default_char_width");
 		for (char c = 32; c < 127; c++) {

src/main/java/nl/andrewlalis/blockbookbinder/view/MainFrame.java

@@ -58,12 +58,8 @@ public class MainFrame extends JFrame {
 	private Container buildContentPane() {
 		JPanel mainPanel = new JPanel(new BorderLayout());
 
-		JPanel doublePanel = new JPanel(new GridLayout(1, 2));
 		BookPreviewPanel bookPreviewPanel = new BookPreviewPanel();
-		doublePanel.add(bookPreviewPanel);
-		SourceTextPanel sourceTextPanel = new SourceTextPanel(bookPreviewPanel);
-		doublePanel.add(sourceTextPanel);
-		mainPanel.add(doublePanel, BorderLayout.CENTER);
+		mainPanel.add(bookPreviewPanel, BorderLayout.CENTER);
 
 		JPanel bottomPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));
 		JButton exportButton = new JButton("Export to Book");

src/main/java/nl/andrewlalis/blockbookbinder/view/SourceTextPanel.java

@@ -1,49 +0,0 @@
-package nl.andrewlalis.blockbookbinder.view;
-
-import nl.andrewlalis.blockbookbinder.control.CleanSourceActionListener;
-import nl.andrewlalis.blockbookbinder.control.ConvertToBookActionListener;
-import nl.andrewlalis.blockbookbinder.view.book.BookPreviewPanel;
-
-import javax.swing.*;
-import javax.swing.border.EmptyBorder;
-import java.awt.*;
-
-/**
- * A panel dedicated to displaying an interacting with a raw source of text for
- * a book.
- */
-public class SourceTextPanel extends JPanel {
-	private final JTextArea textArea;
-
-	public SourceTextPanel(BookPreviewPanel bookPreviewPanel) {
-		super(new BorderLayout());
-
-		this.add(new JLabel("Source Text"), BorderLayout.NORTH);
-		this.setBorder(new EmptyBorder(5, 5, 5, 5));
-
-		this.textArea = new JTextArea();
-		this.textArea.setWrapStyleWord(true);
-		this.textArea.setLineWrap(true);
-		JScrollPane scrollWrappedMainTextArea = new JScrollPane(this.textArea, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
-		this.add(scrollWrappedMainTextArea, BorderLayout.CENTER);
-
-		JPanel rightPanelButtonPanel = new JPanel(new FlowLayout(FlowLayout.LEFT));
-		this.add(rightPanelButtonPanel, BorderLayout.SOUTH);
-
-		JButton convertButton = new JButton("Convert to Book");
-		convertButton.addActionListener(new ConvertToBookActionListener(this, bookPreviewPanel));
-		rightPanelButtonPanel.add(convertButton);
-
-		JButton cleanButton = new JButton("Clean");
-		cleanButton.addActionListener(new CleanSourceActionListener(this));
-		rightPanelButtonPanel.add(cleanButton);
-	}
-
-	public String getSourceText() {
-		return this.textArea.getText();
-	}
-
-	public void setSourceText(String text) {
-		this.textArea.setText(text);
-	}
-}

src/main/java/nl/andrewlalis/blockbookbinder/view/book/BookPageDocumentFilter.java

@@ -1,23 +1,39 @@
 package nl.andrewlalis.blockbookbinder.view.book;
 
+import lombok.Setter;
+import nl.andrewlalis.blockbookbinder.model.BookPage;
+
 import javax.swing.text.AttributeSet;
 import javax.swing.text.BadLocationException;
 import javax.swing.text.DocumentFilter;
 
 public class BookPageDocumentFilter extends DocumentFilter {
+	@Setter
+	private BookPage page;
 
 	@Override
 	public void remove(FilterBypass fb, int offset, int length) throws BadLocationException {
+		System.out.printf("Remove: offset=%d, length=%d\n", offset, length);
+		if (page == null) return;
 		super.remove(fb, offset, length);
 	}
 
 	@Override
 	public void insertString(FilterBypass fb, int offset, String string, AttributeSet attr) throws BadLocationException {
+		System.out.printf("Insert: offset=%d, string=%s\n", offset, string);
+		if (page == null) return;
+		int lineIndex = page.getLineIndexAtOffset(offset);
+		System.out.printf("Insert on line %d: %s\n", lineIndex, page.getLine(lineIndex));
+		fb.getDocument().getStartPosition().getOffset();
 		super.insertString(fb, offset, string, attr);
 	}
 
 	@Override
 	public void replace(FilterBypass fb, int offset, int length, String text, AttributeSet attrs) throws BadLocationException {
+		System.out.printf("Replace: offset=%d, length=%d, text=%s\n", offset, length, text);
+		if (page == null) return;
+		int lineIndex = page.getLineIndexAtOffset(offset);
+		System.out.printf("Replace on line %d: %s\n", lineIndex, page.getLine(lineIndex));
 		super.replace(fb, offset, length, text, attrs);
 	}
 }

src/main/java/nl/andrewlalis/blockbookbinder/view/book/BookPreviewPanel.java

@@ -6,6 +6,7 @@ import nl.andrewlalis.blockbookbinder.model.BookPage;
 
 import javax.swing.*;
 import javax.swing.border.EmptyBorder;
+import javax.swing.text.AbstractDocument;
 import java.awt.*;
 import java.io.IOException;
 import java.io.InputStream;
@@ -19,6 +20,7 @@ public class BookPreviewPanel extends JPanel {
 	private int currentPage = 0;
 
 	private final JTextArea previewPageTextArea;
+	private final BookPageDocumentFilter documentFilter;
 	private final JLabel titleLabel;
 
 	private final JButton previousPageButton;
@@ -34,7 +36,10 @@ public class BookPreviewPanel extends JPanel {
 		this.setBorder(new EmptyBorder(5, 5, 5, 5));
 
 		this.previewPageTextArea = new JTextArea();
-		this.previewPageTextArea.setEditable(false);
+		this.documentFilter = new BookPageDocumentFilter();
+		AbstractDocument doc = (AbstractDocument) this.previewPageTextArea.getDocument();
+		doc.setDocumentFilter(this.documentFilter);
+		this.previewPageTextArea.setEditable(true);
 		try {
 			InputStream is = this.getClass().getClassLoader().getResourceAsStream("fonts/1_Minecraft-Regular.otf");
 			if (is == null) {
@@ -84,7 +89,9 @@ public class BookPreviewPanel extends JPanel {
 		previewButtonPanel.add(this.lastPageButton);
 		this.add(previewButtonPanel, BorderLayout.SOUTH);
 
-		this.setBook(new Book());
+		Book starterBook = new Book();
+		starterBook.addPage(new BookPage());
+		this.setBook(starterBook);
 	}
 
 	private void displayCurrentPage() {
@@ -98,12 +105,12 @@ public class BookPreviewPanel extends JPanel {
 
 	public void setBook(Book book) {
 		this.book = book;
-		this.currentPage = 0;
-		this.displayCurrentPage();
+		this.setCurrentPage(0);
 	}
 
 	public void setCurrentPage(int page) {
 		this.currentPage = page;
+		this.documentFilter.setPage(this.book.getPage(page));
 		this.displayCurrentPage();
 	}
 

src/main/resources/application.properties

@@ -1,6 +1,6 @@
 # Settings for the application's GUI.
 frame.title=Block Book Binder
-frame.default_width=800
+frame.default_width=500
 frame.default_height=600
 
 export_dialog.min_width=400