Really lame code to show text diff using IntelliJ diff tool (this code is for https://github.com/dkandalov/live-plugin)

plugin.groovy

import com.intellij.icons.AllIcons
import com.intellij.openapi.diff.DiffContent
import com.intellij.openapi.diff.DiffRequest
import com.intellij.openapi.diff.DiffTool
import com.intellij.openapi.diff.SimpleContent
import com.intellij.openapi.diff.impl.DiffPanelImpl
import com.intellij.openapi.fileTypes.FileType
import com.intellij.openapi.fileTypes.FileTypes
import com.intellij.openapi.project.Project
import com.intellij.openapi.wm.WindowManager
import com.intellij.openapi.wm.ex.WindowManagerEx

import javax.swing.JComponent
import javax.swing.JFrame
import javax.swing.JPanel
import java.awt.*
import java.awt.event.KeyEvent
import java.awt.event.KeyListener

import static liveplugin.PluginUtil.*

if (isIdeStartup) return

def left = """same text
different text
only on the left side

"""
def right = """same text
a bit different text

only on the right side
"""

showDiff(project, left, right)

def showDiff(Project project, String left, String right, FileType leftFileType = FileTypes.PLAIN_TEXT, FileType rightFileType = FileTypes.PLAIN_TEXT,
            String leftTitle = "", String rightTitle = "", String windowTitle = "") {
    def leftContent = new SimpleContent(left, FileTypes.PLAIN_TEXT)
    def rightContent = new SimpleContent(right, FileTypes.PLAIN_TEXT)

    def diffPanel = new DiffPanelImpl((Window) null, project, (boolean) true, (boolean) true, (int) 0, (DiffTool) null)
    diffPanel.enableToolbar(false)
    diffPanel.requestFocus = false
    diffPanel.diffRequest = new MyDiffRequest(leftContent, rightContent, leftTitle, rightTitle, windowTitle)

    JFrame jframe = new JFrame().with {
        add(new JPanel().with {
            layout = new BorderLayout()
            add(diffPanel.component, BorderLayout.CENTER)
            it
        })
        preferredSize = WindowManagerEx.instance.getFrame(project).size
        pack()
        requestFocus()
        visible = true
        iconImage = WindowManagerEx.instance.getFrame(project)?.iconImage
        it
    }
    addKeyHandlerRecursivelyTo(jframe.rootPane, jframe)
    jframe
}

class MyDiffRequest extends DiffRequest {
    private final leftContent
    private final rightContent
    private final leftTitle
    private final rightTitle
    private final windowTitle

    MyDiffRequest(leftContent, rightContent, leftTitle = "", rightTitle = "", windowTitle = "") {
        super(null)
        this.leftContent = leftContent
        this.rightContent = rightContent
        this.leftTitle = leftTitle
        this.rightTitle = rightTitle
        this.windowTitle = windowTitle
    }

    @Override DiffContent[] getContents() { [leftContent, rightContent] }
    @Override String[] getContentTitles() { [leftTitle, rightTitle] }
    @Override String getWindowTitle() { windowTitle }
}

def addKeyHandlerRecursivelyTo(JComponent component, JFrame jframe) {
    component.addKeyListener(new KeyListener() {
        @Override void keyReleased(KeyEvent keyEvent) {
            if (keyEvent.keyCode == KeyEvent.VK_ESCAPE) {
                show("enter")
                jframe.dispose()
            }
        }
        @Override void keyTyped(KeyEvent e) {}
        @Override void keyPressed(KeyEvent e) {}
    })
    component.components.each{ addKeyHandlerRecursivelyTo(it, jframe) }
}