Fix crash on reverseIndex and cmdRestoreCursor (issue #256) (#431)

* Fix crash on reverseIndex when yBase exceeds lines.count (#256)

The crash occurred because Buffer.clear() did not reset yBase to 0.
When switching between normal and alternate buffers, the stale yBase
value could cause buffer.y + buffer.yBase to exceed buffer.lines.count,
triggering an out-of-bounds access in reverseIndex() and scroll().

Changes:
- Buffer.swift: Add yBase = 0 to clear() to fix the root cause
- Terminal.swift: Add defensive bounds checks in reverseIndex() and
  scroll() to prevent crashes if buffer state becomes invalid
- BufferTests.swift: Add 9 unit tests covering the bug and edge cases

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Fix crash in cmdRestoreCursor when savedX/savedY are invalid

The function was restoring buffer.x and buffer.y from saved values
without bounds checking, causing abort() in Debug builds when savedX
or savedY became invalid after resize/scroll operations.

Fix: Clamp savedX and savedY to valid ranges before assignment:
- buffer.x = min(max(0, buffer.savedX), cols - 1)
- buffer.y = min(max(0, buffer.savedY), rows - 1)

This is the same class of bug as issue #256.

Added 3 tests for cursor restore clamping behavior.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: PhilipLudington

Sources/SwiftTerm/Buffer.swift

@@ -300,16 +300,17 @@ public final class Buffer {
     public func clear ()
     {
         yDisp = 0
+        yBase = 0
         xBase = 0
         linesTop = 0
         x = 0
         y = 0
-        
+
         _lines = CircularBufferLineList (maxLength: getCorrectBufferLength(rows))
         _lines.makeEmpty = { [unowned self] line in getBlankLine(attribute: CharData.defaultAttr, isWrapped: false) }
         scrollTop = 0
         scrollBottom = rows - 1
-        
+
         // Figure out how to do this elegantly
         // SetupTabStops ()
     }

Sources/SwiftTerm/Terminal.swift

@@ -2493,8 +2493,10 @@ open class Terminal {
 
     func cmdRestoreCursor (_ pars: [Int], _ collect: cstring)
     {
-        buffer.x = buffer.savedX
-        buffer.y = buffer.savedY
+        // Clamp savedX and savedY to valid ranges to prevent abort() in Debug builds.
+        // Saved values can become invalid after resize/scroll operations.
+        buffer.x = min(max(0, buffer.savedX), cols - 1)
+        buffer.y = min(max(0, buffer.savedY), rows - 1)
         curAttr = buffer.savedAttr
         charset = buffer.savedCharset
         originMode = buffer.savedOriginMode
@@ -5021,6 +5023,14 @@ open class Terminal {
         } else {
             // scrollTop is non-zero which means no line will be going to the
             // scrollback, instead we can just shift them in-place.
+
+            // Ensure the indices are within bounds to prevent crash (related to issue #256)
+            // This can happen when the buffer has been trimmed and yBase is stale
+            guard bottomRow < buffer.lines.count else {
+                print ("scroll: bottomRow \(bottomRow) >= lines.count \(buffer.lines.count), state: yBase=\(buffer.yBase) scrollTop=\(buffer.scrollTop) scrollBottom=\(buffer.scrollBottom) isAlternate=\(isCurrentBufferAlternate)")
+                return
+            }
+
             let scrollRegionHeight = bottomRow - topRow + 1 /*as it's zero-based*/
             if scrollRegionHeight > 1 {
                 if !buffer.lines.shiftElements (start: topRow + 1, count: scrollRegionHeight - 1, offset: -1) {
@@ -5435,11 +5445,20 @@ open class Terminal {
             // possibly move the code below to term.reverseScroll()
             // test: echo -ne '\e[1;1H\e[44m\eM\e[0m'
             // blankLine(true) is xterm/linux behavior
+            let startIndex = buffer.y + buffer.yBase
             let scrollRegionHeight = buffer.scrollBottom - buffer.scrollTop
-            if !buffer.lines.shiftElements (start: buffer.y + buffer.yBase, count: scrollRegionHeight, offset: 1) {
+
+            // Ensure the start index is within bounds to prevent crash (issue #256)
+            // This can happen when the buffer has been trimmed and yBase is stale
+            guard startIndex < buffer.lines.count else {
+                print ("reverseIndex: start index \(startIndex) >= lines.count \(buffer.lines.count), state: y=\(buffer.y) yBase=\(buffer.yBase) scrollTop=\(buffer.scrollTop) scrollBottom=\(buffer.scrollBottom) isAlternate=\(isCurrentBufferAlternate)")
+                return
+            }
+
+            if !buffer.lines.shiftElements (start: startIndex, count: scrollRegionHeight, offset: 1) {
                 print ("Assertion on reverseIndex, state was: y=\(buffer.y) scrollTop=\(buffer.scrollTop)  yDisp=\(buffer.yDisp) linesTop=\(buffer.linesTop) isAlternate=\(isCurrentBufferAlternate)")
             }
-            buffer.lines [buffer.y + buffer.yBase] = buffer.getBlankLine (attribute: eraseAttr ())
+            buffer.lines [startIndex] = buffer.getBlankLine (attribute: eraseAttr ())
             updateRange (startLine: buffer.scrollTop, endLine: buffer.scrollBottom)
         } else if buffer.y > 0 {
             buffer.y -= 1