Keep the unified diff scrollable when a file is emptied

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/unifieddiff/internal/UnifiedDiffCodeMiningProvider.java

@@ -229,15 +229,7 @@ private void createLineHeaderCodeMinings(List<UnifiedDiff> diffs, List<ICodeMini
 					continue;
 				}
 				try {
-					ICodeMining mining;
-					if (diff.leftStart == doc.getLength()) {
-						mining = new UnifiedDiffFooterCodeMining(doc, this, null, diff, tabWidth,
-								this.deletionBackgroundColor);
-					} else {
-						mining = new UnifiedDiffLineHeaderCodeMining(new Position(diff.leftStart, 1), this, diff,
-								tabWidth, this.detailedDiffColor, this.deletionBackgroundColor, tv);
-					}
-					minings.add(mining);
+					minings.add(createMining(doc, diff, diff.leftStart, tabWidth, tv));
 				} catch (BadLocationException e) {
 					error(e);
 				}
@@ -247,15 +239,7 @@ private void createLineHeaderCodeMinings(List<UnifiedDiff> diffs, List<ICodeMini
 					continue;
 				}
 				try {
-					ICodeMining mining;
-					if (diff.leftStart == doc.getLength()) {
-						mining = new UnifiedDiffFooterCodeMining(doc, this, null, diff, tabWidth,
-								this.deletionBackgroundColor);
-					} else {
-						mining = new UnifiedDiffLineHeaderCodeMining(new Position(diff.leftStart + diff.leftLength, 1),
-								this, diff, tabWidth, this.detailedDiffColor, this.deletionBackgroundColor, tv);
-					}
-					minings.add(mining);
+					minings.add(createMining(doc, diff, diff.leftStart + diff.leftLength, tabWidth, tv));
 				} catch (BadLocationException e) {
 					error(e);
 				}
@@ -264,22 +248,37 @@ private void createLineHeaderCodeMinings(List<UnifiedDiff> diffs, List<ICodeMini
 					continue;
 				}
 				try {
-					ICodeMining mining;
-					if (diff.leftStart == doc.getLength()) {
-						mining = new UnifiedDiffFooterCodeMining(doc, this, null, diff, tabWidth,
-								this.deletionBackgroundColor);
-					} else {
-						mining = new UnifiedDiffLineHeaderCodeMining(new Position(diff.leftStart, 1), this, diff,
-								tabWidth, this.detailedDiffColor, this.deletionBackgroundColor, tv);
-					}
-					minings.add(mining);
+					minings.add(createMining(doc, diff, diff.leftStart, tabWidth, tv));
 				} catch (BadLocationException e) {
 					error(e);
 				}
 			}
 		}
 	}
 
+	/**
+	 * A line header mining reserves its height as the vertical indent of its line,
+	 * so the viewer can scroll over it. A footer mining reserves nothing and is
+	 * therefore only used where no line start is available: behind the last,
+	 * non-empty line of the document.
+	 */
+	private ICodeMining createMining(IDocument doc, UnifiedDiff diff, int offset, int tabWidth, ITextViewer tv)
+			throws BadLocationException {
+		int end = doc.getLength();
+		if (offset >= end && !startsLine(doc, end)) {
+			return new UnifiedDiffFooterCodeMining(doc, this, null, diff, tabWidth, this.deletionBackgroundColor);
+		}
+		// a position must not reach beyond the document, otherwise the annotation model
+		// silently drops it
+		int start = Math.min(offset, end);
+		return new UnifiedDiffLineHeaderCodeMining(new Position(start, start < end ? 1 : 0), this, diff, tabWidth,
+				this.detailedDiffColor, this.deletionBackgroundColor, tv);
+	}
+
+	private static boolean startsLine(IDocument doc, int offset) throws BadLocationException {
+		return doc.getLineOffset(doc.getLineOfOffset(offset)) == offset;
+	}
+
 	static class UnifiedDiffFooterCodeMining extends DocumentFooterCodeMining {
 		private final String unifiedDiffLabel;
 		private final Color deletionBackgroundColor;

team/tests/org.eclipse.team.tests.core/src/org/eclipse/team/tests/ui/UnifiedDiffManagerTest.java

@@ -322,6 +322,25 @@ public void testWhitespaceOnlyChangeIsReportedWhenNotIgnored() {
 				"with ignoreWhiteSpace(false) the changed indentation is a diff");
 	}
 
+	/**
+	 * A diff at the very end of the document is shown as a code mining. Only a line
+	 * header mining reserves its height in the text widget, so an emptied file must
+	 * not fall back to a footer mining: the removed content would be unreachable
+	 * because the viewer would have nothing to scroll over.
+	 */
+	@Test
+	public void testRemovingTheWholeContentStaysScrollable() {
+		setEditorContent("");
+		String removed = "a removed line\n".repeat(200);
+
+		assertTrue(UnifiedDiff.create(editor, removed, UnifiedDiffMode.OVERLAY_READ_ONLY_MODE).open().isOK());
+
+		StyledText widget = viewer().getTextWidget();
+		int reserved = waitForVerticalIndent(widget);
+		assertTrue(reserved >= 200 * widget.getLineHeight(),
+				"the 200 removed lines must reserve scrollable space, but only " + reserved + " pixels were reserved");
+	}
+
 	/**
 	 * Opening a second diff on the same editor must replace the first one instead
 	 * of stacking annotations and diffs on top of each other.
@@ -410,6 +429,20 @@ private static int countAnnotations(IAnnotationModel model, String type) {
 		return annotations(model, type).size();
 	}
 
+	/**
+	 * The vertical indent of a line is only applied while the code mining is
+	 * painted, and the minings themselves are resolved asynchronously.
+	 */
+	private static int waitForVerticalIndent(StyledText widget) {
+		long deadline = System.currentTimeMillis() + 10_000;
+		int indent = 0;
+		while (indent == 0 && System.currentTimeMillis() < deadline) {
+			forcePaintCycle(widget);
+			indent = widget.getLineVerticalIndent(0);
+		}
+		return indent;
+	}
+
 	private static void forcePaintCycle(StyledText tw) {
 		if (tw == null || tw.isDisposed()) {
 			return;