Diagnostics shows how big TuiCode thinks a terminal cell is (#335)

AGENTS.md

@@ -190,6 +190,7 @@ What follows is how TuiCode implements it today.
 - `tui` opens `AboutView`. If the terminal supports sixel (WezTerm, iTerm2, tmux 3.6, Windows Terminal, foot…) it shows the artwork as an image. Otherwise (kitty, Ghostty, Terminal.app — TG 2.1.0 has no kitty graphics) it shows ASCII art.
 - `SixelProbe.Detect` sends DA1 once at startup (`WorkbenchHost` ctor), so the dialog knows up front and shows a spinner, not ASCII art that gets replaced. `SixelProbe.MeasureCell` runs each time About opens, because the cell's pixel size changes when the window moves between a Retina and a non-Retina screen. We don't use TG's `SixelSupportDetector`: it asks `CSI 16 t` first and iTerm2 never answers, which costs TG's 1 s abandon timeout, and its fallback (window pixels ÷ cells) counts the title bar and margins. `MeasureCell` sends iTerm2's `OSC 1337 ; ReportCellSize` and `CSI 16 t` together and takes the first answer. `Detect` also parses tmux's DA1 reply (`…;4c`), which TG's check misses.
 - iTerm2 reports sizes in points everywhere, its `CSI 14 t` reply included. Only `ReportCellSize` carries the scale (`2.0` on Retina), so `ParseIterm2CellSize` multiplies by it (clamped to 1–4; absent in older iTerm2s, meaning 1). Sixels are drawn in device pixels, so ignoring it drew the About image at a quarter of its area (#333).
+- F12 Diagnostics runs `MeasureCell` too each time it opens and shows the size with the reply it came from (`CellMeasurement.Source`), or that none came and 10 × 20 was assumed (#335).
 - The cell size has to be exact: iTerm2 blanks every row an image touches, so an image that ends mid-row leaves a dark band (`AboutImage.Fit` rounds down to whole rows and `Cover` trims the top/bottom to match).
 - Encoding runs in the background and is cached per pixel size, so reopening shows the image immediately.
 - TG re-emits queued sixels on every output write and only rewrites cells whose contents changed, so `AboutView.Dispose` dequeues its sixel and sets `ClearScreenNextIteration`. Otherwise the image stays on screen after close.

src/TuiCode.Workbench/About/SixelProbe.cs

@@ -10,6 +10,10 @@ internal sealed record SixelSupport(bool IsSupported, SizeF CellPixels)
     public static readonly SixelSupport Unsupported = new(false, SizeF.Empty);
 }
 
+internal enum CellSizeSource { Iterm2Report, CellResolutionReply, Assumed }
+
+internal sealed record CellMeasurement(SizeF Pixels, CellSizeSource Source, float Scale = 1);
+
 internal static class SixelProbe
 {
     private static readonly SizeF DefaultCellPixels = new(10, 20);
@@ -29,41 +33,57 @@ public static void Detect(IDriver driver, Action<bool> found)
     // iTerm2 answers only its own query and most other terminals only CSI 16 t. An unanswered query takes TG a
     // second to abandon, so ask both at once and take the first answer.
     /// <summary>Asks how many pixels a cell is.</summary>
-    public static void MeasureCell(IDriver driver, Action<SizeF> found)
+    public static void MeasureCell(IDriver driver, Action<CellMeasurement> found)
+    {
+        if (driver.IsLegacyConsole)
+        {
+            found(new CellMeasurement(DefaultCellPixels, CellSizeSource.Assumed));
+            return;
+        }
+
+        MeasureCell(driver.QueueAnsiRequest, found);
+    }
+
+    internal static void MeasureCell(Action<AnsiEscapeSequenceRequest> queue, Action<CellMeasurement> found)
     {
         var settled = false;
         var misses = 0;
 
-        driver.QueueAnsiRequest(new AnsiEscapeSequenceRequest
+        queue(new AnsiEscapeSequenceRequest
         {
             Request = $"{EscSeqUtils.OSC}1337;ReportCellSize{EscSeqUtils.ST}",
             Value = "1337",
             Terminator = EscSeqUtils.ST,
             ResponseReceived = response => Answer(ParseIterm2CellSize(response)),
             Abandoned = () => Answer(null),
         });
-        Queue(driver, EscSeqUtils.CSI_RequestSixelResolution,
-            response => Answer(ParseCellResolution(response)),
+        Queue(queue, EscSeqUtils.CSI_RequestSixelResolution,
+            response => Answer(ParseCellResolution(response) is { } size
+                ? new CellMeasurement(size, CellSizeSource.CellResolutionReply)
+                : null),
             () => Answer(null));
 
-        void Answer(SizeF? cellPixels)
+        void Answer(CellMeasurement? measurement)
         {
             if (settled) return;
-            if (cellPixels is { } size)
+            if (measurement is not null)
             {
                 settled = true;
-                found(size);
+                found(measurement);
             }
             else if (++misses == 2)
             {
                 settled = true;
-                found(DefaultCellPixels);
+                found(new CellMeasurement(DefaultCellPixels, CellSizeSource.Assumed));
             }
         }
     }
 
     private static void Queue(IDriver driver, AnsiEscapeSequence sequence, Action<string?> received, Action abandoned) =>
-        driver.QueueAnsiRequest(new AnsiEscapeSequenceRequest
+        Queue(driver.QueueAnsiRequest, sequence, received, abandoned);
+
+    private static void Queue(Action<AnsiEscapeSequenceRequest> queue, AnsiEscapeSequence sequence, Action<string?> received, Action abandoned) =>
+        queue(new AnsiEscapeSequenceRequest
         {
             Request = sequence.Request,
             Value = sequence.Value,
@@ -84,12 +104,13 @@ internal static bool IndicatesSixel(string? response) =>
     }
 
     /// <summary>Reads iTerm2's <c>OSC 1337 ; ReportCellSize=height;width;scale ST</c>, in points times the scale.</summary>
-    internal static SizeF? ParseIterm2CellSize(string? response)
+    internal static CellMeasurement? ParseIterm2CellSize(string? response)
     {
         var match = Regex.Match(response ?? "", @"ReportCellSize=([\d.]+);([\d.]+)(?:;([\d.]+))?");
         if (!match.Success || Positive(match.Groups[2].Value, match.Groups[1].Value) is not { } points) return null;
         var scale = float.TryParse(match.Groups[3].Value, CultureInfo.InvariantCulture, out var s) ? Math.Clamp(s, 1, 4) : 1;
-        return scale == 1 ? points : new SizeF(MathF.Round(points.Width * scale), MathF.Round(points.Height * scale));
+        var pixels = scale == 1 ? points : new SizeF(MathF.Round(points.Width * scale), MathF.Round(points.Height * scale));
+        return new CellMeasurement(pixels, CellSizeSource.Iterm2Report, scale);
     }
 
     private static SizeF? Positive(string width, string height) =>

src/TuiCode.Workbench/Diagnostics/DiagnosticsView.cs

@@ -1,5 +1,7 @@
+using System.Globalization;
 using Terminal.Gui.Drivers;
 using TuiCode.Abstractions;
+using TuiCode.Workbench.About;
 using TuiCode.Workbench.Services;
 
 namespace TuiCode.Workbench.Diagnostics;
@@ -16,6 +18,7 @@ public sealed class DiagnosticsView : Window
     private readonly Label _keyBase;
     private readonly Label _keyRune;
     private readonly Label _cursorColour;
+    private readonly Label _cellSize;
     private readonly string? _askedCursorColour;
 
     public IKeybindingService Scope => _scopeKeybindings;
@@ -35,7 +38,8 @@ public DiagnosticsView(string driverName, string kittyNegotiationStatus, string?
         var kittyStatusLines = WrapText(kittyNegotiationStatus, GetFieldWidth());
         var kittyStatusText = string.Join("\n", kittyStatusLines);
         var cursorColourY = 2 + kittyStatusLines.Length;
-        var keyHeadingY = cursorColourY + 2;
+        var cellSizeY = cursorColourY + 1;
+        var keyHeadingY = cellSizeY + 2;
         var nameY = keyHeadingY + 1;
         var hintY = nameY + 6;
         Height = hintY + 6;
@@ -73,6 +77,15 @@ public DiagnosticsView(string driverName, string kittyNegotiationStatus, string?
             Text = cursorColour is null ? "not set by the theme" : $"asked {cursorColour}  •  asking the terminal…",
         };
 
+        var cellSizeHeading = Heading("Cell size", cellSizeY);
+        _cellSize = new Label
+        {
+            X = FieldX,
+            Y = cellSizeY,
+            Width = GetFieldWidth(),
+            Text = "asking the terminal…",
+        };
+
         var keyHeading = Heading("Last key", keyHeadingY);
         var nameLabel = Heading("  Name", nameY);
         var hexLabel = Heading("  Hex", nameY + 1);
@@ -100,6 +113,7 @@ public DiagnosticsView(string driverName, string kittyNegotiationStatus, string?
 
         Add(driverHeading, driverNameLabel, kittyHeading, kittyStatusLabel,
             cursorColourHeading, _cursorColour,
+            cellSizeHeading, _cellSize,
             keyHeading, nameLabel, hexLabel, baseLabel, runeLabel,
             _keyName, _keyHex, _keyBase, _keyRune,
             hint, footer);
@@ -162,6 +176,21 @@ public void ShowTerminalCursorColour(string? reported)
               + (CursorColourProbe.Matches(_askedCursorColour, reported) ? "✓" : "✗");
     }
 
+    public string CellSizeText => _cellSize.Text;
+
+    internal void ShowCellSize(CellMeasurement cell) => _cellSize.Text = DescribeCellSize(cell);
+
+    private static string DescribeCellSize(CellMeasurement cell)
+    {
+        var source = cell.Source switch
+        {
+            CellSizeSource.Iterm2Report => string.Create(CultureInfo.InvariantCulture, $"iTerm2 report, scale {cell.Scale:0.0##}"),
+            CellSizeSource.CellResolutionReply => "CSI 16 t",
+            _ => "no answer, assumed",
+        };
+        return string.Create(CultureInfo.InvariantCulture, $"{cell.Pixels.Width:0.#} × {cell.Pixels.Height:0.#} px  •  {source}");
+    }
+
     public void UpdateLastKey(Key key)
     {
         var raw = (uint)key.KeyCode;

src/TuiCode.Workbench/WorkbenchHost.cs

@@ -1476,9 +1476,9 @@ private void PresentAbout(AboutView view)
 
         view.Present(null);
         // Measured on every open: the window may have moved to a screen with another scale since the last one.
-        SixelProbe.MeasureCell(driver, cellPixels => _app.Invoke(() =>
+        SixelProbe.MeasureCell(driver, cell => _app.Invoke(() =>
         {
-            if (ReferenceEquals(_activeAbout, view)) view.Present(new SixelSupport(true, cellPixels));
+            if (ReferenceEquals(_activeAbout, view)) view.Present(new SixelSupport(true, cell.Pixels));
         }));
     }
 
@@ -2482,11 +2482,18 @@ private void OpenDiagnostics()
         _scopes.Push(view.Scope);
         view.SetFocus();
 
-        if (_cursorColour is not null && _app.Driver is { } driver)
+        if (_app.Driver is not { } driver) return;
+
+        if (_cursorColour is not null)
             CursorColourProbe.Read(driver, reported => _app.Invoke(() =>
             {
                 if (ReferenceEquals(_activeDiagnostics, view)) view.ShowTerminalCursorColour(reported);
             }));
+
+        SixelProbe.MeasureCell(driver, cell => _app.Invoke(() =>
+        {
+            if (ReferenceEquals(_activeDiagnostics, view)) view.ShowCellSize(cell);
+        }));
     }
 
     private string GetKittyNegotiationStatus()

tests/TuiCode.Tests/AboutViewTests.cs

@@ -1,4 +1,5 @@
 using Terminal.Gui.Drawing;
+using Terminal.Gui.Drivers;
 using Terminal.Gui.Views;
 using TuiCode.Workbench.About;
 
@@ -82,7 +83,58 @@ public class SixelProbeTests
     public void ParseIterm2CellSize_reads_the_cell_in_device_pixels(string reply, float width, float height) =>
         Assert.Equal(
             new System.Drawing.SizeF(width, height),
-            SixelProbe.ParseIterm2CellSize($"\x1b]1337;ReportCellSize={reply}\x1b\\"));
+            SixelProbe.ParseIterm2CellSize($"\x1b]1337;ReportCellSize={reply}\x1b\\")?.Pixels);
+
+    [Theory]
+    [InlineData("17.50;8.00;2.0", 2f)]
+    [InlineData("17.50;8.00;1.0", 1f)]
+    [InlineData("17.50;8.00", 1f)]
+    public void ParseIterm2CellSize_reports_the_scale_it_used(string reply, float scale)
+    {
+        var cell = SixelProbe.ParseIterm2CellSize($"\x1b]1337;ReportCellSize={reply}\x1b\\");
+
+        Assert.Equal(CellSizeSource.Iterm2Report, cell?.Source);
+        Assert.Equal(scale, cell?.Scale);
+    }
+
+    [Fact]
+    public void MeasureCell_reports_the_query_that_answered_first()
+    {
+        var requests = new List<AnsiEscapeSequenceRequest>();
+        var found = new List<CellMeasurement>();
+        SixelProbe.MeasureCell(requests.Add, found.Add);
+
+        requests[1].ResponseReceived!("\x1b[6;20;10t");
+        requests[0].ResponseReceived!("\x1b]1337;ReportCellSize=17.5;8.0;2.0\x1b\\");
+
+        Assert.Equal([new CellMeasurement(new System.Drawing.SizeF(10, 20), CellSizeSource.CellResolutionReply)], found);
+    }
+
+    [Fact]
+    public void MeasureCell_takes_the_other_query_when_the_first_goes_unanswered()
+    {
+        var requests = new List<AnsiEscapeSequenceRequest>();
+        var found = new List<CellMeasurement>();
+        SixelProbe.MeasureCell(requests.Add, found.Add);
+
+        requests[1].Abandoned!();
+        requests[0].ResponseReceived!("\x1b]1337;ReportCellSize=17.5;8.0;2.0\x1b\\");
+
+        Assert.Equal([new CellMeasurement(new System.Drawing.SizeF(16, 35), CellSizeSource.Iterm2Report, 2)], found);
+    }
+
+    [Fact]
+    public void MeasureCell_assumes_a_size_when_neither_query_is_answered()
+    {
+        var requests = new List<AnsiEscapeSequenceRequest>();
+        var found = new List<CellMeasurement>();
+        SixelProbe.MeasureCell(requests.Add, found.Add);
+
+        requests[0].Abandoned!();
+        requests[1].Abandoned!();
+
+        Assert.Equal([new CellMeasurement(new System.Drawing.SizeF(10, 20), CellSizeSource.Assumed)], found);
+    }
 
     [Theory]
     [InlineData(null)]

tests/TuiCode.Tests/DiagnosticsViewTests.cs

@@ -1,4 +1,6 @@
+using System.Drawing;
 using Terminal.Gui.Views;
+using TuiCode.Workbench.About;
 using TuiCode.Workbench.Diagnostics;
 
 namespace TuiCode.Tests;
@@ -73,6 +75,47 @@ public void ShowTerminalCursorColour_says_when_the_terminal_did_not_answer()
         Assert.Equal("asked #1F2328  •  terminal didn't answer", view.CursorColourText);
     }
 
+    [Theory]
+    [InlineData(16, 35, "Iterm2Report", 2f, "16 × 35 px  •  iTerm2 report, scale 2.0")]
+    [InlineData(8, 17.5f, "Iterm2Report", 1f, "8 × 17.5 px  •  iTerm2 report, scale 1.0")]
+    [InlineData(12, 26, "Iterm2Report", 1.5f, "12 × 26 px  •  iTerm2 report, scale 1.5")]
+    [InlineData(10, 20, "CellResolutionReply", 1f, "10 × 20 px  •  CSI 16 t")]
+    [InlineData(10, 20, "Assumed", 1f, "10 × 20 px  •  no answer, assumed")]
+    public void ShowCellSize_says_how_big_a_cell_is_and_where_that_came_from(
+        float width, float height, string source, float scale, string expected)
+    {
+        using var view = new DiagnosticsView("ansi", "No");
+
+        view.ShowCellSize(new CellMeasurement(new SizeF(width, height), Enum.Parse<CellSizeSource>(source), scale));
+
+        Assert.Equal(expected, view.CellSizeText);
+    }
+
+    [Theory]
+    [InlineData("No")]
+    [InlineData("Yes (DisambiguateEscapeCodes, ReportEventTypes, ReportAlternateKeys, ReportAllKeysAsEscapeCodes)")]
+    public void Cell_size_row_sits_on_one_line_under_the_cursor_colour_row(string kittyStatus)
+    {
+        using var view = new DiagnosticsView("ansi", kittyStatus, "#1F2328");
+        view.ShowTerminalCursorColour("#FFFFFF");
+        view.ShowCellSize(new CellMeasurement(new SizeF(16, 35), CellSizeSource.Iterm2Report, 2));
+        view.Layout(new Size(200, 100));
+
+        var labels = view.SubViews.OfType<Label>().ToList();
+        var colour = labels.Single(label => label.Text.StartsWith("asked", StringComparison.Ordinal));
+        var heading = labels.Single(label => label.Text == "Cell size");
+        var cell = labels.Single(label => label.Text == view.CellSizeText);
+        var lastKey = labels.Single(label => label.Text == "Last key");
+        var footer = labels.Single(label => label.Text == "Esc  close");
+
+        Assert.Equal(colour.Frame.Bottom, cell.Frame.Y);
+        Assert.Equal(heading.Frame.Y, cell.Frame.Y);
+        Assert.Equal(1, cell.Frame.Height);
+        Assert.True(cell.Frame.Width >= cell.Text.Length);
+        Assert.True(cell.Frame.Bottom < lastKey.Frame.Y);
+        Assert.True(footer.Frame.Bottom <= view.Viewport.Height);
+    }
+
     [Theory]
     [InlineData("No")]
     [InlineData("Yes (DisambiguateEscapeCodes, ReportEventTypes, ReportAlternateKeys, ReportAllKeysAsEscapeCodes)")]