About's image fills the dialog on a Retina screen (#333)

AGENTS.md

@@ -188,7 +188,8 @@ What follows is how TuiCode implements it today.
 ## About dialog (#107)
 
 - `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` runs once at startup (`WorkbenchHost` ctor), so the dialog knows up front and shows a spinner, not ASCII art that gets replaced. 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. We send DA1, then iTerm2's `OSC 1337 ; ReportCellSize` and `CSI 16 t` together and take the first answer. It also parses tmux's DA1 reply (`…;4c`), which TG's check misses.
+- `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).
 - 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

@@ -14,29 +14,22 @@ internal static class SixelProbe
 {
     private static readonly SizeF DefaultCellPixels = new(10, 20);
 
-    /// <summary>Asks the terminal whether it draws sixel images and how many pixels a cell is.</summary>
-    public static void Detect(IDriver driver, Action<SixelSupport> found)
+    /// <summary>Asks the terminal whether it draws sixel images.</summary>
+    public static void Detect(IDriver driver, Action<bool> found)
     {
         if (driver.IsLegacyConsole)
         {
-            found(SixelSupport.Unsupported);
+            found(false);
             return;
         }
 
-        Queue(driver, EscSeqUtils.CSI_SendDeviceAttributes,
-            response =>
-            {
-                if (IndicatesSixel(response))
-                    FindCellPixels(driver, cellPixels => found(new SixelSupport(true, cellPixels)));
-                else
-                    found(SixelSupport.Unsupported);
-            },
-            () => found(SixelSupport.Unsupported));
+        Queue(driver, EscSeqUtils.CSI_SendDeviceAttributes, response => found(IndicatesSixel(response)), () => found(false));
     }
 
     // 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.
-    private static void FindCellPixels(IDriver driver, Action<SizeF> found)
+    /// <summary>Asks how many pixels a cell is.</summary>
+    public static void MeasureCell(IDriver driver, Action<SizeF> found)
     {
         var settled = false;
         var misses = 0;
@@ -90,11 +83,13 @@ internal static bool IndicatesSixel(string? response) =>
         return match.Success ? Positive(match.Groups[2].Value, match.Groups[1].Value) : null;
     }
 
-    /// <summary>Reads iTerm2's <c>OSC 1337 ; ReportCellSize=height;width;scale ST</c>.</summary>
+    /// <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)
     {
-        var match = Regex.Match(response ?? "", @"ReportCellSize=([\d.]+);([\d.]+)");
-        return match.Success ? Positive(match.Groups[2].Value, match.Groups[1].Value) : null;
+        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));
     }
 
     private static SizeF? Positive(string width, string height) =>

src/TuiCode.Workbench/WorkbenchHost.cs

@@ -75,7 +75,7 @@ public sealed class WorkbenchHost : IDisposable
     private AboutView? _activeAbout;
     private DocumentInfoView? _activeDocumentInfo;
     private BlameView? _activeBlame;
-    private SixelSupport? _sixelSupport;
+    private bool? _sixelSupported;
     private MnemonicView? _activeMnemonics;
     private OpenView? _activeOpen;
     private PathPromptView? _activePathPrompt;
@@ -126,10 +126,10 @@ public WorkbenchHost(
         _terminalCursors.Detect();
         // Detected up front so About can show a spinner rather than ASCII art that the image then replaces.
         if (_app.Driver is { } driver)
-            SixelProbe.Detect(driver, support => _app.Invoke(() =>
+            SixelProbe.Detect(driver, supported => _app.Invoke(() =>
             {
-                _sixelSupport = support;
-                _activeAbout?.Present(support);
+                _sixelSupported = supported;
+                if (_activeAbout is { } about) PresentAbout(about);
             }));
         _workbench = workbench;
         _commands = commands;
@@ -1461,7 +1461,23 @@ private void OpenAbout()
         _scopes.Push(view.Scope);
         view.SetFocus();
 
-        view.Present(_sixelSupport);
+        PresentAbout(view);
+    }
+
+    private void PresentAbout(AboutView view)
+    {
+        if (_sixelSupported is not true || _app.Driver is not { } driver)
+        {
+            view.Present(_sixelSupported is null ? null : SixelSupport.Unsupported);
+            return;
+        }
+
+        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(() =>
+        {
+            if (ReferenceEquals(_activeAbout, view)) view.Present(new SixelSupport(true, cellPixels));
+        }));
     }
 
     private void CloseAbout(AboutView view)

tests/TuiCode.Tests/AboutViewTests.cs

@@ -70,18 +70,26 @@ public void Cover_trims_the_top_and_bottom_to_keep_the_aspect_ratio()
 
 public class SixelProbeTests
 {
-    [Fact]
-    public void ParseIterm2CellSize_reads_width_and_height()
-    {
-        var size = SixelProbe.ParseIterm2CellSize("\x1b]1337;ReportCellSize=26.5;11.0;2.0\x1b\\");
-
-        Assert.Equal(new System.Drawing.SizeF(11f, 26.5f), size);
-    }
+    [Theory]
+    [InlineData("17.50;8.00;2.0", 16, 35)]
+    [InlineData("26.0;11.0;2.0", 22, 52)]
+    [InlineData("26.5;11.0;1.0", 11, 26.5)]
+    [InlineData("26.5;11.0", 11, 26.5)]
+    [InlineData("17.5;8.0;1.5", 12, 26)]
+    [InlineData("20;10;400", 40, 80)]
+    [InlineData("20;10;0", 10, 20)]
+    [InlineData("20;10;1.2.3", 10, 20)]
+    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\\"));
 
     [Theory]
     [InlineData(null)]
     [InlineData("\x1b[?64;1;2;4c")]
     [InlineData("\x1b]1337;ReportCellSize=0;11\x1b\\")]
+    [InlineData("\x1b]1337;ReportCellSize=0;11;2.0\x1b\\")]
+    [InlineData("\x1b]1337;ReportCellSize=1.2.3;11;2.0\x1b\\")]
     public void ParseIterm2CellSize_rejects_other_replies(string? response) =>
         Assert.Null(SixelProbe.ParseIterm2CellSize(response));