FULL PRODUCT VERSION :
java version "1.8.0_131"
Java(TM) SE Runtime Environment (build 1.8.0_131-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.131-b11, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 10.0.10586]


A DESCRIPTION OF THE PROBLEM :
With specific (strange) characters, the rendering of text blocks the EDT for a few seconds up to minutes, then works again.
The blocking occurs after calling textArea.setText() or AbstractDocument.insertString().

The specific characters originate from an EBCDIC-encoded file that was read using UTF-8 (I'm aware that's incorrect).
Profiling the app showed that in such a case, a huge amount of char arrays (gigabytes in my app) was created in javax.swing.text.GapContent.getChars(int, int, Segment). Apparently, the code path commented with "// partial return not allowed, must copy" was entered in such a case.

I attached a program that reproduces the problem.
There's a noticeable lag when including a specific string, when not, rendering is instant.
Why I do this? I ran across this on coincidence when writing an app that views several files at once, and the IBM500-encoded zos/jawt_md.h file was included. Everything was read UTF-8.
I'm aware that reading EBCDIC with UTF-8 is wrong, but I still think that those ugly chars should not cause the EDT to block.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Please run the sample program

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
EDT should not block on specific characters
ACTUAL -
EDT blocks, app does not respond at all, for seconds/minutes.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.awt.*;
import javax.swing.*;
import javax.swing.text.*;

public class FooFrame {

static String s0 = "Hello World";
// (truncated) contents of jdk-headers-1.6.0/include/ibm/zos/jawt_md.h file encoding=IBM500/EBCDIC, read with UTF8
static String s1 = "a\\\u0015@\\@~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\u0015@\\@@\ufffd\ufffd\ufffd@\u00d6\ufffd\ufffd\ufffd\ufffd\ufffd\ufffd\ufffd\ufffd\ufffd\ufffd\u0015@\\@@\ufffd\ufffd\ufffd@\u25a4\ufffd\ufffd\ufffd@\u0501\ufffd\ufffd\ufffd\ufffd\ufffd\ufffd\ufffd\u0015@\\\u0015@\\@@\ufffd\ufffd\ufffd@\ufffd\ufffd\ufffdk@\u0441\ufffd\ufffdM\ufffd\ufffd]@\u3143\ufffd\ufffd\ufffd\ufffd\ufffd\ufffd\ufffd@\u0144\ufffd\ufffd\ufffd\ufffd\ufffdk@\ufffd\ufffd\u0015@\\@@M\ufffd]@\u00d6\ufffd\ufffd\ufffd\ufffd\ufffd\ufffd\ufffd@\ufffd\ufffd\ufffd@\u00d6\ufffd\ufffdK@\ufffd\ufffd\ufffd\ufffdk@\ufffd\ufffd\ufffd\ufffdK@\ufffd\ufffd\ufffd@\u0649\ufffd\ufffd\ufffd\ufffd@\u0645\ufffd\ufffd\ufffd\ufffd\ufffd\ufffd\u0015@\\\u0015@\\";

public static void main(String[] args) throws Exception {
EventQueue.invokeAndWait(() -> new FooFrame().go());
}

void go() {
JFrame frame = new JFrame("BugRepro");
frame.setBounds(100, 100, 800, 600);
frame.setContentPane(foo());
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}

JCheckBox c;

JPanel foo() {
JPanel panel = new JPanel(new BorderLayout());
JButton b;
JTextArea area = new JTextArea();
JPanel p = new JPanel(new FlowLayout());
panel.add(p, BorderLayout.NORTH);
p.add(b = new JButton("click me"));
p.add(c = new JCheckBox("include strange chars", true));
panel.add(new JLabel("Hello"), BorderLayout.SOUTH);
panel.add(new JScrollPane(area), BorderLayout.CENTER);

// need to set a new Document b/c setText() sets the text of the old document.
b.addActionListener(e -> area.setDocument(makeDoc()));
return panel;
}

Document makeDoc() {
Document doc = new PlainDocument();
try {
doc.insertString(0, makeString(), new SimpleAttributeSet());
} catch (BadLocationException e) {
throw new Error(e);
}
return doc;
}

String makeString() {
int MAX = 1000;

StringBuilder buf = new StringBuilder();
extracted(MAX, buf);
System.out.println("Including strange chars: " + c.isSelected());
if (c.isSelected()) {
buf.append(s1); // XXX This line slows down the rendering noticeably since it contains strange chars
}
extracted(MAX, buf);

return buf.toString();
}

static void extracted(int MAX, StringBuilder buf) {
for (int i = 0; i < MAX; i++) {
buf.append(s0);
buf.append("\n");
}
}
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Not passing those chars into a text component.