Revert d651e3a to restore BCE behavior. Issue 12723

This reverts commit d651e3a ("Use defaults rather than current SGR
color when scrolling lines into view").

The reverted commit broke BCE (Background Color Erase) compliance,
which vim relies on. When vim sets a background color and scrolls,
it expects scrolled-in lines to use the current SGR background color.
After the reverted commit, scrolled-in lines used terminal defaults
instead, causing inconsistent background colors in vim.

BCE behavior: when new lines scroll into view, they are filled with
the current SGR background color (via grid's defaultChar). This is
synchronized from the terminal before each token executes.

Added unit tests for BCE behavior:
- testBCE_ScrollingUsesDefaultCharBackgroundColor
- testBCE_ScrollRectUsesDefaultCharBackgroundColor

Note: This may reintroduce the behavior reported in issue 12512,
where scrolled-in lines retain their background color after SGR 49
reset. However, this is correct BCE behavior - SGR 49 affects future
operations, not cells already written during scroll.

Author: gnachman

ModernTests/VT100GridTests.swift

@@ -952,7 +952,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 2, height: 2)
-        grid.scroll(rect, downBy: 0, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: 0, softBreak: false)
 
         // Content should be unchanged
         let expectedGrid = [
@@ -981,7 +981,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 2, height: 2)
-        grid.scroll(rect, downBy: 1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: 1, softBreak: false)
 
         let expectedContent = [
             "abcd\n",
@@ -1013,7 +1013,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 2, height: 2)
-        grid.scroll(rect, downBy: -1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -1, softBreak: false)
 
         let expected = [
             "abcd\n",
@@ -1045,7 +1045,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 3, height: 3)
-        grid.scroll(rect, downBy: 2, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: 2, softBreak: false)
 
         let expectedContent = [
             "abcde\n",
@@ -1075,7 +1075,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 3, height: 3)
-        grid.scroll(rect, downBy: -2, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -2, softBreak: false)
 
         let expected = [
             "abcde\n",
@@ -1103,7 +1103,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 3, height: 3)
-        grid.scroll(rect, downBy: 3, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: 3, softBreak: false)
 
         let expected = [
             "abcde\n",
@@ -1131,7 +1131,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 3, height: 3)
-        grid.scroll(rect, downBy: -3, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -3, softBreak: false)
 
         let expected = [
             "abcde\n",
@@ -1159,7 +1159,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 3, height: 3)
-        grid.scroll(rect, downBy: 4, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: 4, softBreak: false)
 
         let expected = [
             "abcde\n",
@@ -1187,7 +1187,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 3, height: 3)
-        grid.scroll(rect, downBy: -4, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -4, softBreak: false)
 
         let expectedContent = [
             "abcde\n",
@@ -1213,7 +1213,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 0, y: 1, width: 3, height: 2)
-        grid.scroll(rect, downBy: -1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -1, softBreak: false)
 
         // The broken EOL_DWC placeholders at rows 0 and 1 should be cleaned up ('.'),
         // blank row inserted at row 2, and the intact DWC_RIGHT at row 3 should remain.
@@ -1250,7 +1250,7 @@ class VT100GridTests: XCTestCase {
 
         // Scroll rectangle x=0,y=1,width=3,height=2 down by 1
         let rect = VT100GridRect(x: 0, y: 1, width: 3, height: 2)
-        grid.scroll(rect, downBy: 1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: 1, softBreak: false)
 
         // After scroll:
         // Row 0: the broken split‐DWC at top is cleaned up → "ab."
@@ -1289,7 +1289,7 @@ class VT100GridTests: XCTestCase {
 
         // Scroll the entire 3×3 region up by 1 (downBy: -1)
         let rect = VT100GridRect(x: 0, y: 0, width: 3, height: 3)
-        grid.scroll(rect, downBy: -1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -1, softBreak: false)
 
         // After scroll:
         // Row 0 <- old Row 1 ("de>\n")
@@ -1325,7 +1325,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 0, width: 3, height: 6)
-        grid.scroll(rect, downBy: -1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -1, softBreak: false)
 
         let expectedContent = [
             "a\0g\0e\n",
@@ -1356,7 +1356,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 0, y: 1, width: 5, height: 3)
-        grid.scroll(rect, downBy: -1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -1, softBreak: false)
 
         let expectedContent = [
             "abcd\n",
@@ -1392,7 +1392,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 0, width: 4, height: 5)
-        grid.scroll(rect, downBy: -1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -1, softBreak: false)
 
         let expectedContent = [
             "a\0fgh\n",
@@ -1423,7 +1423,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 0, y: 0, width: 4, height: 5)
-        grid.scroll(rect, downBy: -1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -1, softBreak: false)
 
         let expectedContent = [
             "e-fg\n",
@@ -1454,7 +1454,7 @@ class VT100GridTests: XCTestCase {
 
         // Empty rect → no-op
         let rect = VT100GridRect(x: 1, y: 1, width: 0, height: 0)
-        grid.scroll(rect, downBy: 1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: 1, softBreak: false)
 
         // Content unchanged
         let expectedContent = [
@@ -1485,7 +1485,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 0, y: 1, width: 3, height: 2)
-        grid.scroll(rect, downBy: -1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -1, softBreak: false)
 
         let expectedContent = [
             "ab\n",
@@ -1514,7 +1514,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 0, y: 1, width: 3, height: 3)
-        grid.scroll(rect, downBy: 1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: 1, softBreak: false)
 
         let expectedContent = [
             "ab\n",
@@ -1543,7 +1543,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 0, y: 1, width: 3, height: 3)
-        grid.scroll(rect, downBy: -1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -1, softBreak: false)
 
         let expectedContent = [
             "ab\n",
@@ -1572,7 +1572,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 0, y: 1, width: 4, height: 3)
-        grid.scroll(rect, downBy: 1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: 1, softBreak: false)
 
         let expectedContent = [
             "abcd\n",
@@ -1602,7 +1602,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 0, y: 1, width: 4, height: 3)
-        grid.scroll(rect, downBy: 2, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: 2, softBreak: false)
 
         let expectedContent = [
             "abcd\n",
@@ -1630,7 +1630,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 0, y: 1, width: 4, height: 3)
-        grid.scroll(rect, downBy: -1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -1, softBreak: false)
 
         let expectedContent = [
             "abcd\n",
@@ -1658,7 +1658,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 0, y: 1, width: 4, height: 3)
-        grid.scroll(rect, downBy: -2, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -2, softBreak: false)
 
         let expectedContent = [
             "abcd\n",
@@ -1686,7 +1686,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 3, height: 3)
-        grid.scroll(rect, downBy: 1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: 1, softBreak: false)
 
         let expectedContent = [
             "abcd",
@@ -1714,7 +1714,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 3, height: 3)
-        grid.scroll(rect, downBy: 2, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: 2, softBreak: false)
 
         let expectedContent = [
             "abcd",
@@ -1745,7 +1745,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 3, height: 3)
-        grid.scroll(rect, downBy: 1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: 1, softBreak: false)
 
         let expectedContent = [
             "abcd",
@@ -1773,7 +1773,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 3, height: 3)
-        grid.scroll(rect, downBy: -1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -1, softBreak: false)
 
         let expectedContent = [
             "abcd",
@@ -1803,7 +1803,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 3, height: 3)
-        grid.scroll(rect, downBy: -1, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -1, softBreak: false)
 
         let expectedContent = [
             "abc\n",
@@ -1831,7 +1831,7 @@ class VT100GridTests: XCTestCase {
         grid.markAllCharsDirty(false, updateTimestamps: false)
 
         let rect = VT100GridRect(x: 1, y: 1, width: 3, height: 3)
-        grid.scroll(rect, downBy: -2, softBreak: false, fill: .zero)
+        grid.scroll(rect, downBy: -2, softBreak: false)
 
         let expectedContent = [
             "abcd",
@@ -3998,6 +3998,93 @@ class VT100GridTests: XCTestCase {
         lineBuffer.removeLastRawLine()
         XCTAssertEqual(lineBuffer.lineStrings, [])
     }
+
+    // MARK: - BCE (Background Color Erase) Tests
+
+    /// Test for issue 12723: When scrolling, newly cleared lines should use
+    /// the grid's defaultChar background color (BCE behavior).
+    func testBCE_ScrollingUsesDefaultCharBackgroundColor() {
+        let width: Int32 = 10
+        let height: Int32 = 4
+        let grid = VT100Grid(size: VT100GridSize(width: width, height: height), delegate: nil)!
+        let lineBuffer = LineBuffer(blockSize: 1000)
+
+        // Fill the grid with text
+        let initialLines = [
+            "line1\n",
+            "line2\n",
+            "line3\n",
+            "line4"
+        ]
+        append(strings: initialLines, toGrid: grid, lineBuffer: lineBuffer)
+
+        // Set up a custom defaultChar with orange 24-bit background color
+        var orangeDefaultChar = screen_char_t()
+        orangeDefaultChar.code = 0
+        orangeDefaultChar.backgroundColorMode = UInt32(ColorMode24bit.rawValue)
+        orangeDefaultChar.backgroundColor = 255  // red
+        orangeDefaultChar.bgGreen = 165
+        orangeDefaultChar.bgBlue = 0
+        grid.defaultChar = orangeDefaultChar
+
+        // Scroll the grid down (which clears the bottom line)
+        grid.cursorX = 0
+        grid.cursorY = 3  // bottom line
+        grid.moveCursorDownOneLineScrolling(into: lineBuffer,
+                                            unlimitedScrollback: true,
+                                            useScrollbackWithRegion: false,
+                                            willScroll: nil,
+                                            sentToLineBuffer: nil)
+
+        // The newly scrolled-in line (line 3, 0-indexed) should have the orange background
+        let line = grid.immutableScreenChars(atLineNumber: 3)!
+
+        // Check that the empty cells have 24-bit orange background
+        XCTAssertEqual(line[0].backgroundColorMode, UInt32(ColorMode24bit.rawValue),
+                       "BCE: scrolled-in line should have 24-bit background color mode from defaultChar")
+        XCTAssertEqual(line[0].backgroundColor, 255,
+                       "BCE: scrolled-in line should have red=255 from defaultChar")
+        XCTAssertEqual(line[0].bgGreen, 165,
+                       "BCE: scrolled-in line should have green=165 from defaultChar")
+        XCTAssertEqual(line[0].bgBlue, 0,
+                       "BCE: scrolled-in line should have blue=0 from defaultChar")
+    }
+
+    /// Test for issue 12723: scrollRect:downBy: should use defaultChar for cleared regions.
+    func testBCE_ScrollRectUsesDefaultCharBackgroundColor() {
+        let grid = gridFromCompactLinesWithContinuationMarks(
+            "abcdef!\n" +
+            "ghijkl!\n" +
+            "mnopqr!\n" +
+            "stuvwx!"
+        )
+
+        // Set up a custom defaultChar with orange 24-bit background color
+        var orangeDefaultChar = screen_char_t()
+        orangeDefaultChar.code = 0
+        orangeDefaultChar.backgroundColorMode = UInt32(ColorMode24bit.rawValue)
+        orangeDefaultChar.backgroundColor = 255  // red
+        orangeDefaultChar.bgGreen = 165
+        orangeDefaultChar.bgBlue = 0
+        grid.defaultChar = orangeDefaultChar
+
+        // Scroll the entire rect up by 1 (this clears the bottom line)
+        let rect = VT100GridRect(origin: VT100GridCoord(x: 0, y: 0),
+                                 size: VT100GridSize(width: grid.size.width, height: grid.size.height))
+        grid.scroll(rect, downBy: -1, softBreak: false)
+
+        // The newly cleared bottom line should have the orange background
+        let line = grid.immutableScreenChars(atLineNumber: 3)!
+
+        XCTAssertEqual(line[0].backgroundColorMode, UInt32(ColorMode24bit.rawValue),
+                       "BCE: scrollRect cleared line should have 24-bit background color mode")
+        XCTAssertEqual(line[0].backgroundColor, 255,
+                       "BCE: scrollRect cleared line should have red=255")
+        XCTAssertEqual(line[0].bgGreen, 165,
+                       "BCE: scrollRect cleared line should have green=165")
+        XCTAssertEqual(line[0].bgBlue, 0,
+                       "BCE: scrollRect cleared line should have blue=0")
+    }
 }
 
 extension VT100Grid {

ModernTests/VT100ScreenTests.swift

@@ -1480,6 +1480,7 @@ class VT100ScreenTests: XCTestCase {
                        ".....!\n" +
                        ".....!")
     }
+
 }
 
 // a very simple LCG-based RNG

sources/VT100Grid.h

@@ -321,10 +321,7 @@ makeCursorLineSoft:(BOOL)makeCursorLineSoft;
 // Clears the left-over region.
 // If |softBreak| is YES then the soft line break on the top line (when scrolling down) or bottom
 // line (when scrolling up) is preserved. Otherwise it is made hard.
-- (void)scrollRect:(VT100GridRect)rect
-            downBy:(int)direction
-         softBreak:(BOOL)softBreak
-          fillChar:(screen_char_t)fillChar;
+- (void)scrollRect:(VT100GridRect)rect downBy:(int)direction softBreak:(BOOL)softBreak;
 
 // Load contents from a DVR frame.
 - (void)setContentsFromDVRFrame:(const screen_char_t*)s