feat: Add mode 2027 (grapheme cluster) support with DECRQM probing#1637
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feat: Add mode 2027 (grapheme cluster) support with DECRQM probing#1637
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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.
- Track enabled state with graphemeClusterModeEnabled field and add getGraphemeClusterMode() accessor - Automatically disable mode 2027 on terminal close if it was enabled - Restrict DECRQM probing to xterm-like terminals to avoid printing garbage on terminals that don't understand DECRQM - Drain partial/unexpected DECRPM responses to prevent leaving stale bytes in the input buffer - Add LineReader Option.GRAPHEME_CLUSTER to auto-enable mode 2027 during readLine() and disable it in doCleanup() Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Summary
Terminal.supportsGraphemeClusterMode()andsetGraphemeClusterMode(boolean)API methods for terminal mode 2027 ("Unicode Core")CSI ? 2027 $ p) instead of guessing from the terminal type string, since terminals like macOS Terminal.app reportxterm-256colorwithout supporting mode 2027AbstractWindowsTerminalto returnfalsesince Windows console doesn't support DECRQM probingCLAUDE.mdwith project conventions for AI assistantsContext
Mode 2027 tells the terminal to use UAX #29 grapheme cluster segmentation instead of per-codepoint
wcwidth()for cursor positioning. This fixes cursor misalignment for multi-codepoint characters like ZWJ emoji sequences (e.g.,👨🌾), flag sequences, and combined characters.Currently supported by Contour, foot, WezTerm, and Ghostty. The DECRQM probing approach ensures JLine correctly detects support regardless of the reported
TERMtype.Test plan
GraphemeClusterModeTesttests pass./mvx mvn test -pl terminal)