This is a fix for the fact that I was treating savedY in two different

ways in the codebase, either a viewport relative value, or a buffer
relative value (0..<rows vs yBase offset).

I compared this to xterm.js which was the original code for this, and
they do not have that discrepancy - either it got fixed there, or I
introduced the bug.

https://gist.github.com/migueldeicaza/fb7470de4b4c9501c7eb660bc56a3ede

Now treats savedY as absolute (yBase-baesd). Also, it now converts
absolute savedY back to viewport row (savedY - yBase) and then clamps
via restrictCursor().

This should be a more comprehensive fix than the one reported in

#467

While auditing I found another difference, softReset now also resets the saved cursor position

Author: migueldeicaza

Sources/SwiftTerm/Buffer.swift

@@ -135,7 +135,7 @@ public final class Buffer {
     public var savedX: Int
 
     /**
-     * This records the saved Y position
+     * This records the saved absolute Y position (yBase + y)
      */
     public var savedY: Int
 
@@ -370,7 +370,7 @@ public final class Buffer {
     public func softReset ()
     {
         savedAttr = CharData.defaultAttr
-        savedY = 0
+        savedY = yBase
         savedX = 0
         savedCharset = CharSets.defaultCharset
         marginRight = cols-1

Sources/SwiftTerm/Terminal.swift

@@ -757,13 +757,8 @@ open class Terminal {
     }
 
     func resizeBuffers(newColumns: Int, newRows: Int) {
-        // correct the savedY cursor to follow changes to y
-        let dy = normalBuffer.savedY - normalBuffer.y
         normalBuffer.resize (newCols: newColumns, newRows: newRows)
-        normalBuffer.savedY = normalBuffer.y + dy
-        
         altBuffer.resize (newCols: newColumns, newRows: newRows)
-
     }
     public func setup (isReset: Bool = false)
     {
@@ -2424,16 +2419,16 @@ open class Terminal {
 
     func cmdRestoreCursor (_ pars: [Int], _ collect: cstring)
     {
-        // Clamp savedX and savedY to valid ranges to prevent abort() in Debug builds.
-        // Saved values can become invalid after resize/scroll operations.
+        // savedY stores the absolute buffer row (yBase + y). Convert back to viewport-relative.
         buffer.x = min(max(0, buffer.savedX), cols - 1)
-        buffer.y = min(max(0, buffer.savedY), rows - 1)
+        buffer.y = max(0, buffer.savedY - buffer.yBase)
         curAttr = buffer.savedAttr
         charset = buffer.savedCharset
         originMode = buffer.savedOriginMode
         setMarginMode(buffer.savedMarginMode)
         setWraparound(buffer.savedWraparound)
         reverseWraparound = buffer.savedReverseWraparound
+        restrictCursor()
     }
 
     //
@@ -2954,7 +2949,7 @@ open class Terminal {
     func cmdSaveCursor (_ pars: [Int], _ collect: cstring)
     {
         buffer.savedX = buffer.x
-        buffer.savedY = buffer.y
+        buffer.savedY = buffer.yBase + buffer.y
         buffer.savedAttr = curAttr
         buffer.savedCharset = charset
         buffer.savedWraparound = wraparound

Tests/SwiftTermTests/BufferTests.swift

@@ -209,6 +209,40 @@ final class BufferTests: TerminalDelegate {
         #expect(terminal.buffer.y == terminal.rows - 1, "y should be clamped to rows-1")
     }
 
+    /// Test that save cursor stores absolute row coordinates (yBase + y).
+    @Test func testSaveCursorStoresAbsoluteSavedY() {
+        let terminal = Terminal(delegate: self, options: TerminalOptions(cols: 80, rows: 6, scrollback: 200))
+
+        for i in 0..<30 {
+            terminal.feed(text: "line \(i)\r\n")
+        }
+        #expect(terminal.buffer.yBase > 0, "Expected yBase > 0 after scrolling")
+
+        terminal.feed(text: "\u{1b}[2;5H")
+        let expectedSavedY = terminal.buffer.yBase + terminal.buffer.y
+        terminal.feed(text: "\u{1b}7")
+
+        #expect(terminal.buffer.savedY == expectedSavedY, "savedY should store absolute row")
+    }
+
+    /// Test that restore cursor translates absolute savedY back into viewport-relative y.
+    @Test func testRestoreCursorUsesAbsoluteSavedY() {
+        let terminal = Terminal(delegate: self, options: TerminalOptions(cols: 80, rows: 6, scrollback: 200))
+
+        for i in 0..<30 {
+            terminal.feed(text: "line \(i)\r\n")
+        }
+        #expect(terminal.buffer.yBase > 0, "Expected yBase > 0 after scrolling")
+
+        let targetViewportRow = 1
+        terminal.buffer.savedX = 0
+        terminal.buffer.savedY = terminal.buffer.yBase + targetViewportRow
+        terminal.feed(text: "\u{1b}[6;10H")
+        terminal.feed(text: "\u{1b}8")
+
+        #expect(terminal.buffer.y == targetViewportRow, "restore should convert saved absolute row to viewport row")
+    }
+
     /// Test that cmdRestoreCursor clamps negative savedY.
     @Test func testRestoreCursorClampsNegativeSavedY() {
         let terminal = Terminal(delegate: self, options: TerminalOptions(cols: 80, rows: 25))
@@ -239,6 +273,21 @@ final class BufferTests: TerminalDelegate {
         #expect(terminal.buffer.x == terminal.cols - 1, "x should be clamped to cols-1")
     }
 
+    /// Test that DECSTR resets savedY to current yBase.
+    @Test func testSoftResetResetsSavedYToYBase() {
+        let terminal = Terminal(delegate: self, options: TerminalOptions(cols: 80, rows: 6, scrollback: 200))
+
+        for i in 0..<30 {
+            terminal.feed(text: "line \(i)\r\n")
+        }
+        #expect(terminal.buffer.yBase > 0, "Expected yBase > 0 after scrolling")
+
+        terminal.buffer.savedY = 0
+        terminal.softReset()
+
+        #expect(terminal.buffer.savedY == terminal.buffer.yBase, "savedY should reset to yBase")
+    }
+
     // MARK: - Additional edge case tests
 
     /// Test that clear() works correctly when called directly on a buffer