Rounds all corners of a JTextComponent's selection highlight polygon and fills it with a translucent background color

• download example.jar
• download src.zip
• repository

Code

class RoundedSelectionHighlightPainter extends DefaultHighlightPainter {
  protected RoundedSelectionHighlightPainter() {
    super(null);
  }

  @Override public void paint(
      Graphics g, int offs0, int offs1, Shape bounds, JTextComponent c) {
    Graphics2D g2 = (Graphics2D) g.create();
    g2.setRenderingHint(
      RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
    int rgba = c.getSelectionColor().getRGB() & 0xFF_FF_FF | (64 << 24);
    g2.setColor(new Color(rgba, true));
    try {
      Area area = getLinesArea(c, offs0, offs1);
      for (Area a : GeomUtils.singularization(area)) {
        List<Point2D> lst = GeomUtils.convertAreaToPoint2DList(a);
        GeomUtils.flatteningStepsOnRightSide(lst, 3d * 2d);
        g2.fill(GeomUtils.convertRoundedPath(lst, 3d));
      }
    } catch (BadLocationException ex) {
      // can't render
      Logger.getGlobal().severe(ex::getMessage);
    }
    g2.dispose();
  }

  private static Area getLinesArea(JTextComponent c, int offs0, int offs1)
      throws BadLocationException {
    TextUI mapper = c.getUI();
    Area area = new Area();
    int cur = offs0;
    do {
      int startOffset = Utilities.getRowStart(c, cur);
      int endOffset = Utilities.getRowEnd(c, cur);
      Rectangle p0 = mapper.modelToView(c, Math.max(startOffset, offs0));
      Rectangle p1 = mapper.modelToView(c, Math.min(endOffset, offs1));
      if (offs1 > endOffset) {
        p1.width += 6;
      }
      addRectToArea(area, p0.union(p1));
      cur = endOffset + 1;
    } while (cur < offs1);
    return area;
  }

  private static void addRectToArea(Area area, Rectangle rect) {
    area.add(new Area(rect));
  }
}

class RoundedSelectionCaret extends DefaultCaret {
  @Override protected HighlightPainter getSelectionPainter() {
    return new RoundedSelectionHighlightPainter();
  }

  @SuppressWarnings("PMD.AvoidSynchronizedAtMethodLevel")
  @Override protected synchronized void damage(Rectangle r) {
    JTextComponent c = getComponent();
    int startOffset = c.getSelectionStart();
    int endOffset = c.getSelectionEnd();
    if (startOffset == endOffset) {
      super.damage(r);
    } else {
      TextUI mapper = c.getUI();
      try {
        Rectangle p0 = mapper.modelToView(c, startOffset);
        Rectangle p1 = mapper.modelToView(c, endOffset);
        int h = (int) (p1.getMaxY() - p0.getMinY());
        c.repaint(new Rectangle(0, p0.y, c.getWidth(), h));
      } catch (BadLocationException ex) {
        UIManager.getLookAndFeel().provideErrorFeedback(c);
      }
    }
  }
}

References

• Rounds all corners of a JTextComponent's selection highlight polygon and fills it with a translucent background color(Japanese)
• editor.roundedSelection - Default settings reference - Visual Studio Code
• Java Swing Tips: Rounding the corners of a rectilinear polygon generated by node selection in a JTree

Change drawing area of selected highlight in JEditorPane

• download example.jar
• download src.zip
• repository

Code

class ParagraphMarkHighlightPainter extends DefaultHighlightPainter {
  protected ParagraphMarkHighlightPainter(Color color) {
    super(color);
  }

  @Override public Shape paintLayer(
      Graphics g, int offs0, int offs1,
      Shape bounds, JTextComponent c, View view) {
    Shape s = super.paintLayer(g, offs0, offs1, bounds, c, view);
    Rectangle r = s.getBounds();
    if (r.width - 1 <= 0) {
      g.fillRect(r.x + r.width, r.y, r.width + r.height / 2, r.height);
    }
    return s;
  }
}

class WholeLineHighlightPainter extends DefaultHighlightPainter {
  protected WholeLineHighlightPainter(Color color) {
    super(color);
  }

  @Override public Shape paintLayer(
      Graphics g, int offs0, int offs1,
      Shape bounds, JTextComponent c, View view) {
    Rectangle rect = bounds.getBounds();
    rect.width = c.getSize().width;
    return super.paintLayer(g, offs0, offs1, rect, c, view);
  }
}

References

• Change drawing area of selected highlight in JEditorPane(Japanese)