content test [nfc]: Use checks-extensions in list-marker tests; explain a bit more

A couple of the extension members added here don't actually end up
getting used (because the "height of one line" check feels like it
benefits from describing the calculation more explicitly).  Still
the fact we're referring to those properties in a test makes a good
prompt to add the extension so future tests can use it.

Author: gnprice

test/flutter_checks.dart

@@ -4,16 +4,20 @@ library;
 import 'package:checks/checks.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
 
 extension PaintChecks on Subject<Paint> {
   Subject<Shader?> get shader => has((x) => x.shader, 'shader');
 }
 
 extension RectChecks on Subject<Rect> {
+  Subject<double> get left => has((d) => d.left, 'left');
   Subject<double> get top => has((d) => d.top, 'top');
+  Subject<double> get right => has((d) => d.right, 'right');
   Subject<double> get bottom => has((d) => d.bottom, 'bottom');
-
+  Subject<double> get width => has((d) => d.width, 'width');
+  Subject<double> get height => has((d) => d.height, 'height');
   // TODO others
 }
 
@@ -36,6 +40,10 @@ extension GlobalKeyChecks<T extends State<StatefulWidget>> on Subject<GlobalKey<
   Subject<T?> get currentState => has((k) => k.currentState, 'currentState');
 }
 
+extension RenderBoxChecks on Subject<RenderBox> {
+  Subject<Size> get size => has((x) => x.size, 'size');
+}
+
 extension IconChecks on Subject<Icon> {
   Subject<IconData?> get icon => has((i) => i.icon, 'icon');
   Subject<Color?> get color => has((i) => i.color, 'color');
@@ -130,6 +138,11 @@ extension InlineSpanChecks on Subject<InlineSpan> {
   Subject<TextStyle?> get style => has((x) => x.style, 'style');
 }
 
+extension RenderParagraphChecks on Subject<RenderParagraph> {
+  Subject<InlineSpan> get text => has((x) => x.text, 'text');
+  Subject<bool> get didExceedMaxLines => has((x) => x.didExceedMaxLines, 'didExceedMaxLines');
+}
+
 extension SizeChecks on Subject<Size> {
   Subject<double> get width => has((x) => x.width, 'width');
   Subject<double> get height => has((x) => x.height, 'height');

test/widgets/content_test.dart

@@ -248,19 +248,24 @@ void main() {
 
     testWidgets('ordered list markers have enough space to render completely', (tester) async {
       await prepareContent(tester, plainContent(ContentExample.orderedListLargeStart.html));
-      final renderParagraph = tester.renderObject(find.textContaining('9999.')) as RenderParagraph;
-      final textHeight = renderParagraph.size.height;
-      final lineHeight = renderParagraph.text.style!.height! * renderParagraph.text.style!.fontSize!;
+      final marker = tester.renderObject(find.textContaining('9999.')) as RenderParagraph;
+      // The marker has the height of just one line of text, not more.
+      final textHeight = marker.size.height;
+      final lineHeight = marker.text.style!.height! * marker.text.style!.fontSize!;
       check(textHeight).equals(lineHeight);
-      check(renderParagraph.didExceedMaxLines).isFalse();
+      // The marker's text didn't overflow to more lines
+      // (and get cut off by a `maxLines: 1`).
+      check(marker).didExceedMaxLines.isFalse();
     });
 
     testWidgets('ordered list markers are end-aligned', (tester) async {
       await prepareContent(tester, plainContent(ContentExample.orderedListLargeStart.html));
       final marker9999 = tester.getRect(find.textContaining('9999.'));
       final marker10000 = tester.getRect(find.textContaining('10000.'));
-      check(marker9999.width != marker10000.width).isTrue();
-      check(marker9999.right).equals(marker10000.right);
+      // The markers are aligned at their right edge...
+      check(marker9999).right.equals(marker10000.right);
+      // ... and not because they somehow happen to have the same width.
+      check(marker9999).width.isLessThan(marker10000.width);
     });
   });