feat: Add mode 2027 (grapheme cluster) support with DECRQM probing

Add Terminal.supportsGraphemeClusterMode() and setGraphemeClusterMode()
to enable UAX #29 grapheme cluster segmentation for correct cursor
positioning of multi-codepoint characters (ZWJ emoji, flags, etc.).

Support is detected at runtime via DECRQM (CSI ? 2027 $ p) rather than
guessing from the terminal type string, since many terminals report
xterm-256color without supporting mode 2027.

Author: gnodet

terminal/src/main/java/org/jline/terminal/Terminal.java

@@ -1341,6 +1341,40 @@ enum MouseTracking {
      */
     boolean trackFocus(boolean tracking);
 
+    /**
+     * Returns whether the terminal supports mode 2027 (grapheme cluster / Unicode Core).
+     *
+     * <p>
+     * Mode 2027 allows the terminal to use UAX #29 grapheme cluster segmentation
+     * instead of per-codepoint {@code wcwidth()} for cursor positioning. This matters
+     * for multi-codepoint characters like ZWJ emoji sequences (e.g., family emoji),
+     * which would otherwise be counted as multiple separate characters.
+     * </p>
+     *
+     * @return {@code true} if the terminal supports mode 2027, {@code false} otherwise
+     * @see #setGraphemeClusterMode(boolean)
+     */
+    default boolean supportsGraphemeClusterMode() {
+        return false;
+    }
+
+    /**
+     * Enables or disables mode 2027 (grapheme cluster / Unicode Core).
+     *
+     * <p>
+     * When enabled, the terminal uses UAX #29 grapheme cluster segmentation for
+     * cursor positioning. This allows multi-codepoint characters like ZWJ emoji
+     * sequences to be treated as single display units.
+     * </p>
+     *
+     * @param enable {@code true} to enable grapheme cluster mode, {@code false} to disable it
+     * @return {@code true} if the operation succeeded, {@code false} otherwise
+     * @see #supportsGraphemeClusterMode()
+     */
+    default boolean setGraphemeClusterMode(boolean enable) {
+        return false;
+    }
+
     /**
      * Returns the color palette for this terminal.
      *

terminal/src/main/java/org/jline/terminal/impl/AbstractTerminal.java

@@ -83,6 +83,7 @@ public abstract class AbstractTerminal implements TerminalExt {
     protected Runnable onClose;
     protected MouseTracking currentMouseTracking = MouseTracking.Off;
     protected volatile boolean closed = false;
+    private Boolean graphemeClusterModeSupported;
 
     public AbstractTerminal(String name, String type) throws IOException {
         this(name, type, null, SignalHandler.SIG_DFL);
@@ -339,6 +340,68 @@ public boolean trackFocus(boolean tracking) {
         }
     }
 
+    @Override
+    public boolean supportsGraphemeClusterMode() {
+        if (graphemeClusterModeSupported == null) {
+            graphemeClusterModeSupported = probeGraphemeClusterMode();
+        }
+        return graphemeClusterModeSupported;
+    }
+
+    /**
+     * Probes the terminal for mode 2027 support using DECRQM.
+     *
+     * <p>Sends {@code CSI ? 2027 $ p} and expects a DECRPM response
+     * {@code CSI ? 2027 ; Ps $ y} where Ps indicates the mode status.</p>
+     *
+     * @return {@code true} if the terminal recognizes mode 2027
+     */
+    private boolean probeGraphemeClusterMode() {
+        if (TYPE_DUMB.equals(type) || TYPE_DUMB_COLOR.equals(type)) {
+            return false;
+        }
+        try {
+            // Send DECRQM query for mode 2027
+            writer().write("\033[?2027$p");
+            writer().flush();
+
+            // Read DECRPM response: ESC [ ? 2 0 2 7 ; Ps $ y
+            long timeout = 100;
+            if (reader().peek(timeout) < 0) {
+                return false;
+            }
+            int[] expected = {'\033', '[', '?', '2', '0', '2', '7', ';'};
+            for (int e : expected) {
+                if (reader().read(timeout) != e) {
+                    return false;
+                }
+            }
+            int ps = reader().read(timeout);
+            if (ps < '0' || ps > '4') {
+                return false;
+            }
+            if (reader().read(timeout) != '$' || reader().read(timeout) != 'y') {
+                return false;
+            }
+            // Ps: 1=set, 2=reset (can be set), 3=permanently set → supported
+            // Ps: 0=not recognized, 4=permanently reset → not supported
+            return ps == '1' || ps == '2' || ps == '3';
+        } catch (IOException e) {
+            return false;
+        }
+    }
+
+    @Override
+    public boolean setGraphemeClusterMode(boolean enable) {
+        if (supportsGraphemeClusterMode()) {
+            writer().write(enable ? "\033[?2027h" : "\033[?2027l");
+            writer().flush();
+            return true;
+        } else {
+            return false;
+        }
+    }
+
     protected void checkInterrupted() throws InterruptedIOException {
         if (Thread.interrupted()) {
             throw new InterruptedIOException();

terminal/src/main/java/org/jline/terminal/impl/AbstractWindowsTerminal.java

@@ -674,6 +674,11 @@ public void processInputChar(char c) throws IOException {
         slaveInputPipe.write(c);
     }
 
+    @Override
+    public boolean supportsGraphemeClusterMode() {
+        return false;
+    }
+
     @Override
     public boolean trackMouse(MouseTracking tracking) {
         this.tracking = tracking;