Blatant flickering

[yenneferofvengerberg]

Information

• OS: Windows 10/11
• Version: 0.48.0
• Terminal: Dos Prompt, Powershell and Windows Terminal

Describe the bug
Displaying a task list with spinners causes the whole task list to flicker. Apparently, see below, this is a regression introduced in 0.48.0

To Reproduce
The following code exhibits the issue: (tested on .net 6.0 and .net 8.0)

using Spectre.Console;

namespace TestConsole
{
    internal static class Program
    {
        private static async Task LoopAsync(ProgressTask task)
        {
            await Task.Yield();

            for (int i = 0; i < 100; i++)
            {
                task.Increment(1);
                await Task.Delay(TimeSpan.FromSeconds(4)).ConfigureAwait(false);
            }
        }

        private static async Task Main()
        {
            Console.WriteLine("Flickering Test");

            var progress = AnsiConsole.Progress()
              .AutoRefresh(true)
              .AutoClear(false)
              .HideCompleted(false)
              .Columns(
              [
                  new TaskDescriptionColumn(),
                  new ProgressBarColumn(),
                  new PercentageColumn(),
                  new RemainingTimeColumn(),
                  new SpinnerColumn(Spinner.Known.Dots),
              ]);

            await progress.StartAsync(async x =>
            {
                var tasks = new List<Task>();
                for (int i = 0; i < 10; i++)
                {
                    var t = x.AddTask($"Task {i}", true, 100);
                    tasks.Add(LoopAsync(t));
                }

                await Task.WhenAll(tasks).ConfigureAwait(false);
            }).ConfigureAwait(false);
        }
    }
}

Expected behavior
No flickering, like in 0.47.0

Screenshots
0.48.0:

0.47.0:

Additional context
The "regression" has been introduced in the commit 71a5d83

The offending line is in src/Spectre.Console/Live/LiveRenderable.cs:

-            return new ControlCode("\r" + CUU(linesToMoveUp));
+            return new ControlCode("\r" + (EL(2) + CUU(1)).Repeat(linesToMoveUp));

[ShaunLawrie]

This is also happening for other widget types in 0.48 including up to at least 0.48.1-preview.0.32 in Windows terminal

flicker.mp4

[GOATS2K]

This happens in progress displays without spinners as well.

[phil-scott-78]

Can confirm what @yenneferofvengerberg posted, rolling back that commit related to long lines smooths things out again.

Probably need something a bit more clever than brute forcing a clear on each line each render. I'd almost be inclined to suggest using the RenderHook that now exists on LiveRenderables as the mechanism for including extra info while a progress is running rather than write line. Gives us a bit more control over the whole process too

[patriksvensson]

Is there a PR to roll this back?

[yenneferofvengerberg]

I considered doing it, but I couldn't figure out how to maintain the correct behavior when the text goes out of screen. If someone can provide me a hint, I would be happy to try

[patriksvensson]

Since it's a regression I would say that it's OK with restoring it how it was, and look at finding a solution for it later.

[phil-scott-78]

Opened a PR for the revert #1504

Regarding the original issue with WriteLine during a LiveRenderable, I wonder if some cleverness could be done to detect whether or not we can do the overwrite the content (no-flicker) vs erase what was written. Have to keep state of what was previously there on the last write, but I think it would be doable if someone had the gumption.