[web] Fix placeholder-only paragraphs

lib/web_ui/lib/src/engine/text/layout_service.dart

@@ -107,6 +107,8 @@ class TextLayoutService {
       // *** THE MAIN MEASUREMENT PART *** //
       // ********************************* //
 
+      final isLastSpan = spanIndex == spanCount - 1;
+
       if (span is PlaceholderSpan) {
         if (currentLine.widthIncludingSpace + span.width <= constraints.width) {
           // The placeholder fits on the current line.
@@ -119,6 +121,11 @@ class TextLayoutService {
           }
           currentLine.addPlaceholder(span);
         }
+
+        if (isLastSpan) {
+          lines.add(currentLine.build());
+          break;
+        }
       } else if (span is FlatTextSpan) {
         spanometer.currentSpan = span;
         final LineBreakResult nextBreak = currentLine.findNextBreak(span.end);
@@ -219,17 +226,23 @@ class TextLayoutService {
         minIntrinsicWidth = widthOfLastSegment;
       }
 
+      // Max intrinsic width includes the width of trailing spaces.
+      if (maxIntrinsicWidth < currentLine.widthIncludingSpace) {
+        maxIntrinsicWidth = currentLine.widthIncludingSpace;
+      }
+
       if (currentLine.end.isHard) {
-        // Max intrinsic width includes the width of trailing spaces.
-        if (maxIntrinsicWidth < currentLine.widthIncludingSpace) {
-          maxIntrinsicWidth = currentLine.widthIncludingSpace;
-        }
         currentLine = currentLine.nextLine();
       }
 
       // Only go to the next span if we've reached the end of this span.
-      if (currentLine.end.index >= span.end && spanIndex < spanCount - 1) {
-        span = paragraph.spans[++spanIndex];
+      if (currentLine.end.index >= span.end) {
+        if (spanIndex < spanCount - 1) {
+          span = paragraph.spans[++spanIndex];
+        } else {
+          // We reached the end of the last span in the paragraph.
+          break;
+        }
       }
     }
   }
@@ -632,7 +645,10 @@ class LineSegment {
   double get widthOfTrailingSpace => widthIncludingSpace - width;
 
   /// Whether this segment is made of only white space.
-  bool get isSpaceOnly => start.index == end.indexWithoutTrailingSpaces;
+  ///
+  /// We rely on the [width] to determine this because relying on incides
+  /// doesn't work well for placeholders (they are zero-length strings).
+  bool get isSpaceOnly => width == 0;
 }
 
 /// Builds instances of [EngineLineMetrics] for the given [paragraph].

lib/web_ui/test/text/layout_service_rich_test.dart

@@ -149,4 +149,22 @@ void testMain() async {
       l('ipsum', 6, 11, hardBreak: true, width: 50.0, left: 0.0),
     ]);
   });
+
+  test('should handle placeholder-only paragraphs', () {
+    final EngineParagraphStyle paragraphStyle = EngineParagraphStyle(
+      fontFamily: 'ahem',
+      fontSize: 10,
+      textAlign: ui.TextAlign.center,
+    );
+    final CanvasParagraph paragraph = rich(paragraphStyle, (builder) {
+      builder.addPlaceholder(300.0, 50.0, ui.PlaceholderAlignment.baseline, baseline: ui.TextBaseline.alphabetic);
+    })..layout(constrain(500.0));
+
+    expect(paragraph.maxIntrinsicWidth, 300.0);
+    expect(paragraph.minIntrinsicWidth, 300.0);
+    expect(paragraph.height, 50.0);
+    expectLines(paragraph, [
+      l('', 0, 0, hardBreak: false, width: 300.0, left: 100.0),
+    ]);
+  });
 }