Fix origin-mode cursor positioning

FIXES.md

@@ -0,0 +1,112 @@
+# Origin mode and scroll region fixes
+
+## Summary
+
+This branch fixes DEC origin-mode cursor positioning with scroll regions. These changes are core terminal emulator behavior and are not specific to Termrig, Avalonia, ConPTY, or any host renderer.
+
+The fixed behavior is:
+
+- `DECSTBM` / `CSI t;b r` moves the cursor to home after setting the scroll region.
+- `CUP` / `CSI row;col H` and `HVP` / `CSI row;col f` treat row coordinates as relative to the scroll region when `DECOM` / origin mode is enabled.
+- `VPA` / `CSI row d` applies the same origin-mode row translation.
+- enabling origin mode moves the cursor to the top margin of the scroll region; disabling origin mode moves the cursor to absolute home.
+
+## Why
+
+Full-screen and prompt-oriented terminal applications often reserve a bottom input or status row by setting a scroll region for the output area. They then use origin-mode cursor addressing inside that region.
+
+If the emulator treats those row coordinates as absolute screen rows, application output can be written outside the intended scroll region. In real-world terminal UIs this can leave stale prompt/status rows in scrollback or place rewritten content on the wrong line.
+
+## Files changed
+
+- `src/XTerm.NET/InputHandler.cs`
+  - Added shared row translation for origin-mode cursor addressing.
+  - Applied that translation to `CUP` / `HVP` and `VPA`.
+  - Homed the cursor after `DECSTBM`.
+  - Homed to the top margin when origin mode is enabled.
+- `src/XTerm.NET.Tests/InputHandlerTests.cs`
+  - Added regression coverage for scroll-region cursor homing.
+  - Added regression coverage for origin-relative `CUP` / `HVP`.
+  - Added regression coverage for origin-relative `VPA`.
+- `src/XTerm.NET.Tests/ModeHandlingTests.cs`
+  - Added regression coverage for enabling origin mode with a non-zero top margin.
+
+## Minimal reproductions
+
+### Scroll region homes the cursor
+
+```csharp
+var terminal = new Terminal(new TerminalOptions { Cols = 20, Rows = 5 });
+var handler = new InputHandler(terminal);
+terminal.Buffer.SetCursor(10, 10);
+
+var parameters = new Params();
+parameters.AddParam(2);
+parameters.AddParam(4);
+handler.HandleCsi("r", parameters);
+
+Assert.Equal(0, terminal.Buffer.X);
+Assert.Equal(0, terminal.Buffer.Y);
+```
+
+### Origin-mode `CUP` is relative to the scroll region
+
+```csharp
+var terminal = new Terminal(new TerminalOptions { Cols = 20, Rows = 5 });
+var handler = new InputHandler(terminal);
+terminal.Buffer.SetScrollRegion(1, 3);
+terminal.OriginMode = true;
+
+var parameters = new Params();
+parameters.AddParam(3);
+parameters.AddParam(20);
+handler.HandleCsi("H", parameters);
+
+Assert.Equal(19, terminal.Buffer.X);
+Assert.Equal(3, terminal.Buffer.Y);
+```
+
+### Origin-mode `VPA` is relative to the scroll region
+
+```csharp
+var terminal = new Terminal(new TerminalOptions { Cols = 20, Rows = 5 });
+var handler = new InputHandler(terminal);
+terminal.Buffer.SetScrollRegion(1, 3);
+terminal.OriginMode = true;
+terminal.Buffer.SetCursor(10, 1);
+
+var parameters = new Params();
+parameters.AddParam(3);
+handler.HandleCsi("d", parameters);
+
+Assert.Equal(10, terminal.Buffer.X);
+Assert.Equal(3, terminal.Buffer.Y);
+```
+
+## Not included
+
+This branch intentionally does not include:
+
+- host-rendering changes
+- PTY or ConPTY line-ending policy
+- Avalonia integration changes
+- Termrig-specific output normalization
+- Docker Compose cell-width fixes from the earlier Docker progress branch
+
+Those are separate concerns. This branch is limited to standard VT scroll-region and origin-mode semantics in XTerm.NET.
+
+## Validation
+
+Run from this repository root:
+
+```powershell
+dotnet test src/XTerm.NET.slnx --no-restore
+```
+
+Result on this branch:
+
+```text
+Passed: 589
+Failed: 0
+Skipped: 0
+```

src/XTerm.NET.Tests/InputHandlerTests.cs

@@ -150,6 +150,26 @@ public void HandleCsi_CursorPosition_SetsCursorPosition()
         Assert.Equal(9, terminal.Buffer.Y);  // 10 - 1 (1-based to 0-based)
     }
 
+    [Fact]
+    public void HandleCsi_CursorPosition_UsesScrollRegionOriginInOriginMode()
+    {
+        // Arrange
+        var terminal = CreateTerminal();
+        var handler = new InputHandler(terminal);
+        terminal.Buffer.SetScrollRegion(4, 19);
+        terminal.OriginMode = true;
+        var params_ = new Params();
+        params_.AddParam(3);
+        params_.AddParam(20);
+
+        // Act
+        handler.HandleCsi("H", params_);
+
+        // Assert
+        Assert.Equal(19, terminal.Buffer.X);
+        Assert.Equal(6, terminal.Buffer.Y);
+    }
+
     [Fact]
     public void HandleCsi_EraseInDisplay_ClearBelow()
     {
@@ -344,6 +364,45 @@ public void HandleCsi_SetScrollRegion_SetsRegion()
         Assert.Equal(19, terminal.Buffer.ScrollBottom); // 20 - 1 (1-based to 0-based)
     }
 
+    [Fact]
+    public void HandleCsi_SetScrollRegion_MovesCursorHome()
+    {
+        // Arrange
+        var terminal = CreateTerminal();
+        var handler = new InputHandler(terminal);
+        terminal.Buffer.SetCursor(10, 10);
+        var params_ = new Params();
+        params_.AddParam(5);
+        params_.AddParam(20);
+
+        // Act
+        handler.HandleCsi("r", params_);
+
+        // Assert
+        Assert.Equal(0, terminal.Buffer.X);
+        Assert.Equal(0, terminal.Buffer.Y);
+    }
+
+    [Fact]
+    public void HandleCsi_SetScrollRegion_MovesCursorToTopMarginInOriginMode()
+    {
+        // Arrange
+        var terminal = CreateTerminal();
+        var handler = new InputHandler(terminal);
+        terminal.OriginMode = true;
+        terminal.Buffer.SetCursor(10, 10);
+        var params_ = new Params();
+        params_.AddParam(5);
+        params_.AddParam(20);
+
+        // Act
+        handler.HandleCsi("r", params_);
+
+        // Assert
+        Assert.Equal(0, terminal.Buffer.X);
+        Assert.Equal(4, terminal.Buffer.Y);
+    }
+
     [Fact]
     public void HandleEsc_Index_MovesDownOrScrolls()
     {
@@ -1115,6 +1174,26 @@ public void HandleCsi_LinePositionAbsolute_MovesToRow()
         Assert.Equal(11, terminal.Buffer.Y); // 12 - 1 = 11 (0-based)
     }
 
+    [Fact]
+    public void HandleCsi_LinePositionAbsolute_UsesScrollRegionOriginInOriginMode()
+    {
+        // Arrange
+        var terminal = CreateTerminal();
+        var handler = new InputHandler(terminal);
+        terminal.Buffer.SetScrollRegion(4, 19);
+        terminal.OriginMode = true;
+        terminal.Buffer.SetCursor(15, 5);
+        var params_ = new Params();
+        params_.AddParam(3);
+
+        // Act
+        handler.HandleCsi("d", params_);
+
+        // Assert
+        Assert.Equal(15, terminal.Buffer.X);
+        Assert.Equal(6, terminal.Buffer.Y);
+    }
+
     [Fact]
     public void HandleCsi_SaveRestoreCursorAnsi_SavesAndRestoresPosition()
     {

src/XTerm.NET.Tests/ModeHandlingTests.cs

@@ -170,6 +170,24 @@ public void SetMode_OriginMode_EnablesOriginMode()
         Assert.Equal(0, terminal.Buffer.Y);
     }
 
+    [Fact]
+    public void SetMode_OriginMode_MovesCursorToTopMargin()
+    {
+        // Arrange
+        var terminal = CreateTerminal();
+        terminal.Buffer.SetScrollRegion(4, 19);
+        terminal.Buffer.SetCursor(10, 10);
+        Assert.False(terminal.OriginMode);
+
+        // Act - DEC private mode
+        terminal.Write($"\x1B[?{(int)TerminalMode.Origin}h");
+
+        // Assert
+        Assert.True(terminal.OriginMode);
+        Assert.Equal(0, terminal.Buffer.X);
+        Assert.Equal(4, terminal.Buffer.Y);
+    }
+
     [Fact]
     public void ResetMode_OriginMode_DisablesOriginMode()
     {

src/XTerm.NET/InputHandler.cs

@@ -847,6 +847,7 @@ private void CursorPosition(Params parameters)
     {
         var row = Math.Max(parameters.GetParam(0, 1), 1) - 1;
         var col = Math.Max(parameters.GetParam(1, 1), 1) - 1;
+        row = GetAbsoluteCursorRow(row);
         _buffer.SetCursor(col, row);
     }
 
@@ -1009,17 +1010,7 @@ private void LinePositionAbsolute(Params parameters)
     {
         // VPA - Line Position Absolute (CSI d)
         var row = Math.Max(parameters.GetParam(0, 1), 1) - 1;
-
-        // Respect origin mode
-        if (_terminal.OriginMode)
-        {
-            row = Math.Clamp(row, _buffer.ScrollTop, _buffer.ScrollBottom);
-        }
-        else
-        {
-            row = Math.Clamp(row, 0, _terminal.Rows - 1);
-        }
-
+        row = GetAbsoluteCursorRow(row);
         _buffer.SetCursor(_buffer.X, row);
     }
 
@@ -1283,6 +1274,23 @@ private void SetScrollRegion(Params parameters)
         var top = Math.Max(parameters.GetParam(0, 1), 1) - 1;
         var bottom = Math.Max(parameters.GetParam(1, _terminal.Rows), 1) - 1;
         _buffer.SetScrollRegion(top, bottom);
+        MoveCursorToHome();
+    }
+
+    private int GetAbsoluteCursorRow(int row)
+    {
+        if (_terminal.OriginMode)
+        {
+            return Math.Clamp(_buffer.ScrollTop + row, _buffer.ScrollTop, _buffer.ScrollBottom);
+        }
+
+        return Math.Clamp(row, 0, _terminal.Rows - 1);
+    }
+
+    private void MoveCursorToHome()
+    {
+        var row = _terminal.OriginMode ? _buffer.ScrollTop : 0;
+        _buffer.SetCursor(0, row);
     }
 
     private void WindowManipulation(Params parameters)
@@ -1540,7 +1548,7 @@ private void SetCSIMode(int mode, bool isPrivate)
 
                 case TerminalMode.Origin:
                     _terminal.OriginMode = true;
-                    _buffer.SetCursor(0, 0);
+                    MoveCursorToHome();
                     break;
 
                 case TerminalMode.Wraparound:
@@ -1729,7 +1737,7 @@ private void ResetCSIMode(int mode, bool isPrivate)
 
                 case TerminalMode.Origin:
                     _terminal.OriginMode = false;
-                    _buffer.SetCursor(0, 0);
+                    MoveCursorToHome();
                     break;
 
                 case TerminalMode.Wraparound: