fix: incorrect multiline autoindent (#1002)

Fixes #1000 

Open to suggestions. I feel like there is no very good way of do this.
This is the best I could come up with.
If we want to preserve the relative indent, then we would just need to
handle the midline paste case.

Let me know what you think

Thanks!

Author: joeljohansson99

org.eclipse.tm4e.languageconfiguration.tests/src/main/java/org/eclipse/tm4e/languageconfiguration/tests/TestIndentationRules.java

@@ -58,6 +58,18 @@ public void testIndentAdjustmentOnPaste() throws Exception {
 		text.insert("function bar() {\n}");
 		assertThat(text.getText()).isEqualTo("public class Foo {\n\tfunction bar() {\n\t}\n}");
 
+		// insert a code snippet with unindented first line and expect the snippet to be correctly indented
+		text.setText("public class Foo {\n\n}");
+		text.setSelection(19);
+		text.insert("function bar() {\n\t\t\n\t}");
+		assertThat(text.getText()).isEqualTo("public class Foo {\n\tfunction bar() {\n\t\t\n\t}\n}");
+
+		// insert an else code snippet with unindented first line after the if code snippet and expect the snippet to be correctly indented
+		text.setText("public class Foo {\n\tfunction foo() {\n\t\tif (cond) {\n\t\t} \n\t}\n}");
+		text.setSelection(55);
+		text.insert("else {\n\t\t}");
+		assertThat(text.getText()).isEqualTo("public class Foo {\n\tfunction foo() {\n\t\tif (cond) {\n\t\t} else {\n\t\t}\n\t}\n}");
+
 		// insert single line text and ensure the text is only indented in blank lines
 		text.setText("public class Foo {\n\n}");
 		text.setSelection(19);

org.eclipse.tm4e.languageconfiguration/src/main/java/org/eclipse/tm4e/languageconfiguration/internal/LanguageConfigurationAutoEditStrategy.java

@@ -163,17 +163,36 @@ && isFollowedBy(doc, command.offset, charPair.close))) {
 
 						final var newIndent = registry.getGoodIndentForLine(doc, lineIndex, contentType, IIndentConverter.of(cursorCfg));
 						if (newIndent != null) {
+							final var normalizedIndent = cursorCfg.normalizeIndentation(newIndent);
 							final var lineStartOffset = doc.getLineOffset(lineIndex);
-
-							// check if the content was pasted into a line while the cursor was not at the beginning of the line
-							// but inside or at the end of an existing line indentation
 							final var offsetInLine = command.offset - lineStartOffset;
-							if (offsetInLine > 0 && doc.get(lineStartOffset, offsetInLine).isBlank()) {
-								command.offset = lineStartOffset;
-								command.length += offsetInLine;
+							final int firstNewline = command.text.indexOf('\n');
+
+							if (firstNewline >= 0) {
+								final var firstLine = command.text.substring(0, firstNewline + 1);
+								// Re-indent lines after the first line
+								final var reindentedRest = TextUtils.replaceIndent(
+										command.text.substring(firstNewline + 1), cursorCfg.indentSize, normalizedIndent, false);
+
+								if (offsetInLine > 0 && !doc.get(lineStartOffset, offsetInLine).isBlank()) {
+									// Content was pasted midline after non-whitespace
+									// First pasted line continues existing content, don't replace indentation
+									command.text = firstLine + reindentedRest;
+								} else {
+									// Content was pasted at the start of the line, or preceded only by whitespace
+									// Replace any existing leading whitespace with the correct indent
+									if (offsetInLine > 0) {
+										command.offset = lineStartOffset;
+										command.length += offsetInLine;
+									}
+									command.text = normalizedIndent + firstLine.replaceFirst("^[ \\t]+", "") + reindentedRest;
+								}
+							} else {
+								// Single-line paste: straightforward indent replacement
+								command.text = TextUtils
+										.replaceIndent(command.text, cursorCfg.indentSize, normalizedIndent, false)
+										.toString();
 							}
-							command.text = TextUtils.replaceIndent(command.text, cursorCfg.indentSize,
-									cursorCfg.normalizeIndentation(newIndent), false).toString();
 							command.shiftsCaret = true;
 						}
 					}