Fixed: Use current bg color when scrolling with horizontal margins

Fixes termux/termux-packages#12556

Issue was also reported here:
https://www.reddit.com/r/termux/comments/1df1dii/how_can_i_fix_this_annoying_screenfilling_thing/

Author: fornwall

terminal-emulator/src/main/java/com/termux/terminal/TerminalEmulator.java

@@ -2092,13 +2092,14 @@ private void setCursorPosition(int x, int y) {
 
     private void scrollDownOneLine() {
         mScrollCounter++;
+        long currentStyle = getStyle();
         if (mLeftMargin != 0 || mRightMargin != mColumns) {
             // Horizontal margin: Do not put anything into scroll history, just non-margin part of screen up.
             mScreen.blockCopy(mLeftMargin, mTopMargin + 1, mRightMargin - mLeftMargin, mBottomMargin - mTopMargin - 1, mLeftMargin, mTopMargin);
             // .. and blank bottom row between margins:
-            mScreen.blockSet(mLeftMargin, mBottomMargin - 1, mRightMargin - mLeftMargin, 1, ' ', mEffect);
+            mScreen.blockSet(mLeftMargin, mBottomMargin - 1, mRightMargin - mLeftMargin, 1, ' ', currentStyle);
         } else {
-            mScreen.scrollDownOneLine(mTopMargin, mBottomMargin, getStyle());
+            mScreen.scrollDownOneLine(mTopMargin, mBottomMargin, currentStyle);
         }
     }
 

terminal-emulator/src/test/java/com/termux/terminal/ScrollRegionTest.java

@@ -127,4 +127,31 @@ public void testScrollRegionDoesNotLimitCursorMovement() {
 				"   xxx"
 			);
 	}
+
+	/**
+	 * See <a href="https://github.com/termux/termux-packages/issues/12556">reported issue</a>.
+	 */
+	public void testClearingWhenScrollingWithMargins() {
+		int newForeground = 2;
+		int newBackground = 3;
+		int size = 3;
+		TerminalTestCase terminal = withTerminalSized(size, size)
+			// Enable horizontal margin and set left margin to 1:
+			.enterString("\033[?69h\033[2s")
+			// Set foreground and background color:
+			.enterString("\033[" + (30 + newForeground) + ";" + (40 + newBackground) + "m")
+			// Enter newlines to scroll down:
+			.enterString("\r\n\r\n\r\n\r\n\r\n");
+		for (int row = 0; row < size; row++) {
+			for (int col = 0; col < size; col++) {
+				// The first column (outside of the scrolling area, due to us setting a left scroll
+				// margin of 1) should be unmodified, the others should use the current style:
+				int expectedForeground = col == 0 ? TextStyle.COLOR_INDEX_FOREGROUND : newForeground;
+				int expectedBackground = col == 0 ? TextStyle.COLOR_INDEX_BACKGROUND : newBackground;
+				terminal.assertForegroundColorAt(row, col, expectedForeground);
+				terminal.assertBackgroundColorAt(row, col, expectedBackground);
+			}
+		}
+	}
+
 }

terminal-emulator/src/test/java/com/termux/terminal/TerminalTestCase.java

@@ -301,6 +301,11 @@ public void assertForegroundColorAt(int externalRow, int column, int color) {
 		assertEquals(color, TextStyle.decodeForeColor(style));
 	}
 
+	public void assertBackgroundColorAt(int externalRow, int column, int color) {
+		long style = mTerminal.getScreen().mLines[mTerminal.getScreen().externalToInternalRow(externalRow)].getStyle(column);
+		assertEquals(color, TextStyle.decodeBackColor(style));
+	}
+
 	public TerminalTestCase assertColor(int colorIndex, int expected) {
 		int actual = mTerminal.mColors.mCurrentColors[colorIndex];
 		if (expected != actual) {