Fix text box orientation in right to left translation mode

src/ui/Features/Main/Layout/InitListViewAndEditBox.cs

@@ -118,6 +118,7 @@ public static Grid MakeLayoutListViewAndEditBox(MainView mainPage, MainViewModel
         var notNullConverter = new NotNullConverter();
         var nullToOpacityConverter = new NullToOpacityConverter();
         var syntaxHighlightingConverter = new TextWithSubtitleSyntaxHighlightingConverter();
+        var textToFlowDirectionConverter = new TextToFlowDirectionConverter();
         vm.SubtitleDataGridSyntaxHighlighting = syntaxHighlightingConverter;
         // How the Text/Original cells fit their text to the window (feature #11590). Read once here;
         // the grid is rebuilt when settings are applied, so a changed mode takes effect then.
@@ -305,6 +306,7 @@ public static Grid MakeLayoutListViewAndEditBox(MainView mainPage, MainViewModel
                 {
                     VerticalAlignment = VerticalAlignment.Center,
                     [!TextBlock.InlinesProperty] = new Binding(nameof(SubtitleLineViewModel.Text)) { Converter = syntaxHighlightingConverter, Mode = BindingMode.OneWay },
+                    [!TextBlock.FlowDirectionProperty] = new Binding(nameof(SubtitleLineViewModel.Text)) { Converter = textToFlowDirectionConverter, Mode = BindingMode.OneWay },
                 };
                 SubtitleGridTextDisplayModeDisplay.ApplyTo(textBlock, gridTextDisplayMode);
 
@@ -336,6 +338,7 @@ public static Grid MakeLayoutListViewAndEditBox(MainView mainPage, MainViewModel
                 {
                     VerticalAlignment = VerticalAlignment.Center,
                     [!TextBlock.InlinesProperty] = new Binding(nameof(SubtitleLineViewModel.OriginalText)) { Converter = syntaxHighlightingConverter, Mode = BindingMode.OneWay },
+                    [!TextBlock.FlowDirectionProperty] = new Binding(nameof(SubtitleLineViewModel.OriginalText)) { Converter = textToFlowDirectionConverter, Mode = BindingMode.OneWay },
                 };
                 SubtitleGridTextDisplayModeDisplay.ApplyTo(textBlock, gridTextDisplayMode);
 
@@ -1250,6 +1253,9 @@ public static Grid MakeLayoutListViewAndEditBox(MainView mainPage, MainViewModel
         // Right panel for text editing (show/duration is to the left)
         var textEditGrid = new Grid
         {
+            // RightToLeftHelper mirrors this grid by name so the current/original
+            // text boxes keep matching the mirrored subtitle grid columns.
+            Name = "SubtitleTextEditGrid",
             ColumnDefinitions = new ColumnDefinitions("*,*,Auto"),
             RowDefinitions = new RowDefinitions("Auto,*,Auto"),
         };
@@ -1733,6 +1739,7 @@ private static Avalonia.Controls.Control MakeTextBox(MainViewModel vm)
             textBox.AddHandler(InputElement.PointerPressedEvent, (_, e) => vm.StoreTextEditorPointerArgs(e), RoutingStrategies.Tunnel);
 
             SetupMacContextMenuForTextBox(textBox, vm);
+            MainHelpers.RightToLeftHelper.FollowContentDirection(textBox);
 
             vm.EditTextBox = new TextBoxWrapper(textBox);
             return textBox;
@@ -1843,6 +1850,7 @@ private static Avalonia.Controls.Control MakeTextBoxOriginal(MainViewModel vm)
             }
 
             SetupMacContextMenuForTextBox(textBox, vm);
+            MainHelpers.RightToLeftHelper.FollowContentDirection(textBox);
 
             vm.EditTextBoxOriginal = new TextBoxWrapper(textBox);
             return textBox;

src/ui/Features/Main/Layout/TextEditorBindingHelper.cs

@@ -5,6 +5,7 @@
 using Avalonia.Media;
 using AvaloniaEdit;
 using Nikse.SubtitleEdit.Features.Shared.TextBoxUtils;
+using Nikse.SubtitleEdit.Logic.Config;
 
 namespace Nikse.SubtitleEdit.Features.Main.Layout;
 
@@ -157,6 +158,14 @@ private void OnTextEditorTextChanged(object? sender, System.EventArgs e)
         UpdateViewModelFromEditor();
         var routedEvent = _isOriginal ? OriginalTextChangedEvent : TextChangedEvent;
         _vm.SubtitleTextChanged(sender, new TextChangedEventArgs(routedEvent));
+
+        // Follow the content: an original subtitle in a left to right language
+        // stays readable next to a right to left working language and vice versa.
+        var requestedDirection = Se.Settings.Appearance.RightToLeft
+            ? FlowDirection.RightToLeft
+            : FlowDirection.LeftToRight;
+        _textEditor.TextArea.FlowDirection = MainHelpers.RightToLeftHelper
+            .GetContentDirection(_textEditor.Text, requestedDirection);
     }
 
     private void OnTextEditorTapped(object? sender, RoutedEventArgs e)

src/ui/Features/Main/MainHelpers/RightToLeftHelper.cs

@@ -1,9 +1,11 @@
-using Avalonia;
+using System.Linq;
+using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Media;
 using Avalonia.VisualTree;
 using AvaloniaEdit.Editing;
 using Nikse.SubtitleEdit.Controls;
+using Nikse.SubtitleEdit.Core.Common;
 using Nikse.SubtitleEdit.Logic.Config;
 
 namespace Nikse.SubtitleEdit.Features.Main.MainHelpers;
@@ -16,6 +18,104 @@ internal static void SetRightToLeftForDataGridAndText(Window window)
         SetFlowDirectionRecursive(window, flowDirection);
     }
 
+    /// <summary>
+    /// A text control follows its content in either mode: text with right to left
+    /// letters (Arabic, Hebrew, and so on) flows right to left, text in a left to
+    /// right script (for example a Turkish or English original subtitle) flows
+    /// left to right. The layout itself still follows only the mode toggle; this
+    /// governs text direction inside a control. Empty controls keep the requested
+    /// direction so typing starts on the side matching the active mode.
+    /// </summary>
+    internal static FlowDirection GetContentDirection(string? text, FlowDirection requested)
+    {
+        if (string.IsNullOrWhiteSpace(text))
+        {
+            return requested;
+        }
+
+        return LanguageAutoDetect.ContainsRightToLeftLetter(text)
+            ? FlowDirection.RightToLeft
+            : FlowDirection.LeftToRight;
+    }
+
+    /// <summary>
+    /// Keeps a text box's flow direction following its content while right to left
+    /// mode is enabled: right to left content stays right to left, left to right
+    /// content (for example a Turkish original next to an Arabic working language)
+    /// aligns left to right. Attach once when the text box is created; text set
+    /// while right to left mode is off is left alone.
+    /// </summary>
+    internal static void FollowContentDirection(TextBox textBox)
+    {
+        textBox.PropertyChanged += (_, e) =>
+        {
+            if (e.Property == TextBox.TextProperty)
+            {
+                var requested = Se.Settings.Appearance.RightToLeft
+                    ? FlowDirection.RightToLeft
+                    : FlowDirection.LeftToRight;
+                textBox.FlowDirection = GetContentDirection(textBox.Text, requested);
+            }
+        };
+    }
+
+    /// <summary>
+    /// Mirrors the main text edit grid so the current and original text boxes keep
+    /// matching the mirrored subtitle grid columns (issue #12249). Setting
+    /// FlowDirection on a grid does not rearrange its children, so the columns are
+    /// swapped for real: the column definitions are reversed (the original column's
+    /// width binding travels with its definition object) and every child is
+    /// remapped to the mirrored column index. Idempotent via the grid's Tag.
+    /// </summary>
+    private static void MirrorTextEditGrid(Grid grid, FlowDirection flowDirection)
+    {
+        var wantMirrored = flowDirection == FlowDirection.RightToLeft;
+        var isMirrored = Equals(grid.Tag, "mirrored");
+        if (wantMirrored == isMirrored)
+        {
+            return;
+        }
+
+        grid.Tag = wantMirrored ? "mirrored" : null;
+
+        var definitions = grid.ColumnDefinitions.ToList();
+        grid.ColumnDefinitions.Clear();
+        definitions.Reverse();
+        foreach (var definition in definitions)
+        {
+            grid.ColumnDefinitions.Add(definition);
+        }
+
+        var last = definitions.Count - 1;
+        foreach (var child in grid.Children)
+        {
+            Grid.SetColumn(child, last - Grid.GetColumn(child));
+        }
+    }
+
+    /// <summary>
+    /// Re-runs the subtitle grid's cell bindings after the right to left mode
+    /// changed: the per line flow direction converter reads the setting, and
+    /// bindings do not re-evaluate on their own when a setting changes. Rebinding
+    /// the items source re-creates the rows; selection and scroll position are
+    /// restored.
+    /// </summary>
+    internal static void RefreshDataGridBindings(DataGrid? grid, System.Collections.IEnumerable? itemsSource, object? selected)
+    {
+        if (grid == null)
+        {
+            return;
+        }
+
+        grid.ItemsSource = null;
+        grid.ItemsSource = itemsSource;
+        if (selected != null)
+        {
+            grid.SelectedItem = selected;
+            grid.ScrollIntoView(selected, null);
+        }
+    }
+
     private static void SetFlowDirectionRecursive(Visual visual, FlowDirection flowDirection)
     {
         if (visual is ComboBox)
@@ -42,13 +142,17 @@ private static void SetFlowDirectionRecursive(Visual visual, FlowDirection flowD
         {
             dataGrid.FlowDirection = flowDirection;
         }
+        else if (visual is Grid grid && grid.Name == "SubtitleTextEditGrid")
+        {
+            MirrorTextEditGrid(grid, flowDirection);
+        }
         else if (visual is TextBox textBox)
         {
-            textBox.FlowDirection = flowDirection;
+            textBox.FlowDirection = GetContentDirection(textBox.Text, flowDirection);
         }
         else if (visual is TextArea textArea)
         {
-            textArea.FlowDirection = flowDirection;
+            textArea.FlowDirection = GetContentDirection(textArea.Document?.Text, flowDirection);
         }
 
         foreach (var child in visual.GetVisualChildren())

src/ui/Features/Main/MainViewModel.cs

@@ -11065,6 +11065,7 @@ private void RightToLeftToggle()
                 Se.Settings.Appearance.RightToLeft = !Se.Settings.Appearance.RightToLeft;
                 IsRightToLeftEnabled = Se.Settings.Appearance.RightToLeft;
                 RightToLeftHelper.SetRightToLeftForDataGridAndText(Window);
+                RightToLeftHelper.RefreshDataGridBindings(SubtitleGrid, Subtitles, SelectedSubtitle);
                 return;
             }
 

src/ui/Logic/ValueConverters/TextToFlowDirectionConverter.cs

@@ -0,0 +1,40 @@
+using Avalonia.Data.Converters;
+using Avalonia.Media;
+using Nikse.SubtitleEdit.Core.Common;
+using Nikse.SubtitleEdit.Logic.Config;
+using System;
+using System.Globalization;
+
+namespace Nikse.SubtitleEdit.Logic.ValueConverters;
+
+/// <summary>
+/// Gives a text presenter the flow direction of its content in either mode: cells
+/// with right to left letters flow right to left (also as the original next to a
+/// left to right working language), cells in a left to right script flow left to
+/// right (also as the original next to a right to left working language). Empty
+/// cells follow the active mode.
+/// </summary>
+public class TextToFlowDirectionConverter : IValueConverter
+{
+    public object Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        var rightToLeftMode = Se.Settings.Appearance.RightToLeft;
+        if (value is string text && !string.IsNullOrWhiteSpace(text))
+        {
+            return LanguageAutoDetect.ContainsRightToLeftLetter(text)
+                ? FlowDirection.RightToLeft
+                : FlowDirection.LeftToRight;
+        }
+
+        // Always produce an explicit direction: a binding that yields UnsetValue
+        // does not fall back to the inherited direction but to the property
+        // default (left to right), which left right to left cells misaligned in
+        // right to left mode.
+        return rightToLeftMode ? FlowDirection.RightToLeft : FlowDirection.LeftToRight;
+    }
+
+    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        throw new NotImplementedException();
+    }
+}