Fix computation of the indentation in DefaultStickyLinesProvider

Fixes: #3254

Author: wahlbrink

bundles/org.eclipse.ui.editors/src/org/eclipse/ui/internal/texteditor/stickyscroll/DefaultStickyLinesProvider.java

@@ -13,7 +13,6 @@
  *******************************************************************************/
 package org.eclipse.ui.internal.texteditor.stickyscroll;
 
-import java.util.Collections;
 import java.util.LinkedList;
 import java.util.List;
 
@@ -35,8 +34,6 @@ public class DefaultStickyLinesProvider implements IStickyLinesProvider {
 
 	private final static int IGNORE_LINE_INDENTATION= -1;
 
-	private final static String TAB= "\t"; //$NON-NLS-1$
-
 	private StickyLinesProperties fProperties;
 
 	@Override
@@ -101,13 +98,22 @@ private String getPreviousContentLine(int startFromLine, StyledText styledText)
 	}
 
 	private int getIndentation(String line) {
-		if (line == null || line.isBlank()) {
-			return IGNORE_LINE_INDENTATION;
+		if (line != null && !line.isBlank()) {
+			int indent = 0;
+			for (int i = 0; i < line.length(); i++) {
+				switch (line.charAt(i)) {
+				case ' ':
+					indent++;
+					continue;
+				case '\t':
+					indent += fProperties.tabWith() - (indent % fProperties.tabWith());
+					continue;
+				default:
+					return indent;
+				}
+			}
 		}
-		String tabAsSpaces= String.join("", Collections.nCopies(fProperties.tabWith(), " ")); //$NON-NLS-1$ //$NON-NLS-2$
-
-		line= line.replace(TAB, tabAsSpaces);
-		return line.length() - line.stripLeading().length();
+		return IGNORE_LINE_INDENTATION;
 	}
 
 	private int mapLineNumberToWidget(ISourceViewer sourceViewer, int line) {

tests/org.eclipse.ui.editors.tests/src/org/eclipse/ui/internal/texteditor/stickyscroll/DefaultStickyLinesProviderTest.java

@@ -137,6 +137,23 @@ public void testIgnoreEmptyLines() {
 		assertEquals(2, stickyLines.get(1).getLineNumber());
 	}
 
+	@Test
+	public void testIgnoreOtherBlankLines() {
+		String text = """
+				line 1
+				    \t
+				 line 2
+				\f
+				  line 3<""";
+		textWidget.setText(text);
+
+		List<IStickyLine> stickyLines = stickyLinesProvider.getStickyLines(sourceViewer, 4, stickyLinesProperties);
+
+		assertEquals(2, stickyLines.size());
+		assertEquals(0, stickyLines.get(0).getLineNumber());
+		assertEquals(2, stickyLines.get(1).getLineNumber());
+	}
+
 	@Test
 	public void testLinesWithTabs() {
 		stickyLinesProperties = new StickyLinesProperties(2, editorPart);
@@ -153,6 +170,24 @@ public void testLinesWithTabs() {
 		assertEquals(1, stickyLines.get(1).getLineNumber());
 	}
 
+	@Test
+	public void testLinesWithTabsAndSpaces() {
+		// tab witdh 4
+		String text = """
+				line 1
+				   \tline 2
+				\t  line 3
+				\t\tline 4<""";
+		textWidget.setText(text);
+
+		List<IStickyLine> stickyLines = stickyLinesProvider.getStickyLines(sourceViewer, 3, stickyLinesProperties);
+
+		assertEquals(3, stickyLines.size());
+		assertEquals(0, stickyLines.get(0).getLineNumber());
+		assertEquals(1, stickyLines.get(1).getLineNumber());
+		assertEquals(2, stickyLines.get(2).getLineNumber());
+	}
+
 	@Test
 	public void testStartAtEmptyLineWithNext() {
 		String text = """