TerminalUI.display() resets JLine's Display every redraw, defeating frame diffing (full-screen rewrite per frame) #1361

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1. github-actions

   added

   status/need-triageTeam needs to triage and take a first lookTeam needs to triage and take a first look

   on Jun 21, 2026
2. czpilar commented on Jun 23, 2026

   

   czpilar

   on Jun 23, 2026

   Last edited by czpilar

   Contributor

   More actions

   Reproduced, and confirmed it affects Windows too.

   TerminalUI.display() calls display.reset() before every display.update() in full screen mode. reset() empties JLine's cached previous frame (oldLines), so the per-line diff has nothing to compare against and re-emits the whole screen on every redraw.

   Verified with a test that measures bytes written for a second, identical frame: current code rewrites everything, whereas keeping JLine's cache writes ~0 bytes. It's purely about bytes written, so it's platform-independent - the test passes on Windows. The visible tearing/flicker additionally depends on the terminal's synchronized-output support.

   Fix: only clear()/reset()/resize() when the terminal size actually changes; otherwise let JLine diff. PR to follow. (Scoped to full screen, as reported; the non-full-screen path uses relative output where the per-frame reset is harder to remove safely.)

   Fixed in PR #1362

   Here is a reproducer, just use tui command to get to full screen mode:

   package org.springframework.shell.samples.helloworld.boot;
   
   import java.util.concurrent.Executors;
   import java.util.concurrent.ScheduledExecutorService;
   import java.util.concurrent.TimeUnit;
   import java.util.concurrent.atomic.AtomicInteger;
   
   import org.springframework.beans.factory.annotation.Autowired;
   import org.springframework.boot.SpringApplication;
   import org.springframework.boot.autoconfigure.SpringBootApplication;
   import org.springframework.shell.core.command.annotation.Command;
   import org.springframework.shell.core.command.annotation.Option;
   import org.springframework.shell.jline.tui.component.message.ShellMessageBuilder;
   import org.springframework.shell.jline.tui.component.view.TerminalUI;
   import org.springframework.shell.jline.tui.component.view.TerminalUIBuilder;
   import org.springframework.shell.jline.tui.component.view.control.BoxView;
   import org.springframework.shell.jline.tui.component.view.event.EventLoop;
   import org.springframework.shell.jline.tui.component.view.event.KeyEvent.Key;
   import org.springframework.shell.jline.tui.geom.HorizontalAlign;
   import org.springframework.shell.jline.tui.geom.VerticalAlign;
   
   @SpringBootApplication
   public class SpringShellApplication {
   
   	@Autowired
   	private TerminalUIBuilder terminalUiBuilder;
   
   	public static void main(String[] args) {
   		SpringApplication.run(SpringShellApplication.class, args);
   	}
   
   	@Command(name = "hello", description = "Say hello to a given name", group = "Greetings",
   			help = "A command that greets the user with 'Hello ${name}!'. Usage: hello [-n | --name]=<name>")
   	public void sayHello(@Option(shortName = 'n', longName = "name", description = "the name of the person to greet",
   			defaultValue = "World") String name) {
   		System.out.println("Hello " + name + "!");
   	}
   
   	// gh-1361 visual demo (throwaway, not for the fix PR): a full-screen view that
   	// redraws on a 100ms timer. Before the fix the whole screen is cleared+rewritten
   	// every tick (the border flickers); after the fix only the counter cell changes.
   	@Command(name = "tui", description = "Full-screen redraw demo (press 'q' to quit)")
   	public void tui() {
   		TerminalUI ui = terminalUiBuilder.build();
   		BoxView view = new BoxView();
   		ui.configure(view);
   		view.setShowBorder(true);
   		view.setTitle("gh-1361 redraw demo - resize the window, press 'q' to quit");
   		AtomicInteger frame = new AtomicInteger();
   		view.setDrawFunction((screen, rect) -> {
   			screen.writerBuilder()
   				.build()
   				.text("redraw #" + frame.get(), rect, HorizontalAlign.CENTER, VerticalAlign.CENTER);
   			return rect;
   		});
   		ui.setRoot(view, true);
   		EventLoop eventLoop = ui.getEventLoop();
   		eventLoop.keyEvents().subscribe(event -> {
   			if (event.getPlainKey() == Key.q) {
   				eventLoop.dispatch(ShellMessageBuilder.ofInterrupt());
   			}
   		});
   		ScheduledExecutorService exec = Executors.newSingleThreadScheduledExecutor();
   		exec.scheduleAtFixedRate(() -> {
   			frame.incrementAndGet();
   			ui.redraw();
   		}, 100, 100, TimeUnit.MILLISECONDS);
   		try {
   			ui.run();
   		}
   		finally {
   			exec.shutdownNow();
   		}
   	}
   
   }
3. fmbenhassine

   added

   type/bugIs a bug reportIs a bug report

   and removed

   status/need-triageTeam needs to triage and take a first lookTeam needs to triage and take a first look

   on Sep 21, 2026
4. fmbenhassine

   added this to the 4.0.4 milestone on Sep 21, 2026
5. fmbenhassine

   closed this as completedin #1362on Sep 21, 2026
6. fmbenhassine

   added

   type/enhancementIs an enhancement requestIs an enhancement request

   and removed

   type/bugIs a bug reportIs a bug report

   on Sep 21, 2026
7. czpilar

   added a commit that references this issue on Sep 21, 2026

   Preserve frame diffing on full screen redraw

   ...

   This issue will close once commit 4cda1a8 is merged into the 'main' branch.

   4cda1a8