Fix tabs to not overwrite cells

app/build.gradle

@@ -18,8 +18,8 @@ android {
         applicationId "com.termux"
         minSdkVersion 21
         targetSdkVersion 23
-        versionCode 29
-        versionName "0.29"
+        versionCode 30
+        versionName "0.30"
     }
 
     buildTypes {

app/src/main/java/com/termux/terminal/TerminalEmulator.java

@@ -474,15 +474,18 @@ public final class TerminalEmulator {
 			else
 				mSession.onBell();
 			break;
-		case 8: // BS
+        case 8: // Backspace (BS, ^H).
             setCursorCol(Math.max(mLeftMargin, mCursorCol - 1));
             break;
-		case 9: // Horizontal tab - move to next tab stop, but not past edge of screen
-			int nextTabStop = nextTabStop(1);
-			while (mCursorCol < nextTabStop) {
-				// Emit newlines to get background color right.
-				processCodePoint(' ');
-			}
+        case 9: // Horizontal tab (HT, \t) - move to next tab stop, but not past edge of screen
+		    // XXX: Should perhaps use color if writing to new cells. Try with
+		    //       printf "\033[41m\tXX\033[0m\n"
+		    // The OSX Terminal.app colors the spaces from the tab red, but xterm does not.
+		    // Note that Terminal.app only colors on new cells, in e.g.
+		    //       printf "\033[41m\t\r\033[42m\tXX\033[0m\n"
+		    // the first cells are created with a red background, but when tabbing over
+		    // them again with a green background they are not overwritten.
+            mCursorCol = nextTabStop(1);
             break;
 		case 10: // Line feed (LF, \n).
 		case 11: // Vertical tab (VT, \v).

app/src/test/java/com/termux/terminal/CursorAndScreenTest.java

@@ -163,7 +163,12 @@ public class CursorAndScreenTest extends TerminalTestCase {
 		}
 	}
 
-	public void testHorizontalTabColorsBackground() {
+    /**
+     * See comments on horizontal tab handling in TerminalEmulator.java.
+     *
+     * We do not want to color already written cells when tabbing over them.
+     */
+	public void DISABLED_testHorizontalTabColorsBackground() {
 		withTerminalSized(10, 3).enterString("\033[48;5;15m").enterString("\t");
 		assertCursorAt(0, 8);
 		for (int i = 0; i < 10; i++) {

app/src/test/java/com/termux/terminal/TerminalTest.java

@@ -258,4 +258,9 @@ public class TerminalTest extends TerminalTestCase {
 		withTerminalSized(3, 3).enterString("abc\r ").assertLinesAre(" bc", "   ", "   ").assertCursorAt(0, 1);
 	}
 
+    public void testTab() {
+        withTerminalSized(11, 2).enterString("01234567890\r\tXX").assertLinesAre("01234567XX0", "           ");
+        withTerminalSized(11, 2).enterString("01234567890\033[44m\r\tXX").assertLinesAre("01234567XX0", "           ");
+    }
+
 }