Fix math blocks after paragraphs

Fixes microsoft#258288

Author: mjbvz

src/vs/workbench/contrib/markdown/browser/markedKatexSupport.ts

@@ -207,6 +207,9 @@ export namespace MarkedKatexExtension {
 		return {
 			name: 'blockKatex',
 			level: 'block',
+			start(src: string) {
+				return src.match(new RegExp(blockRule.source, 'm'))?.index;
+			},
 			tokenizer(src: string, tokens: marked.Token[]) {
 				const match = src.match(blockRule);
 				if (match) {

src/vs/workbench/contrib/markdown/test/browser/__snapshots__/Markdown_Katex_Support_Test_Should_support_blocks_immediately_after_paragraph.0.snap

@@ -0,0 +1 @@
+<p>Block example:</p><span class="katex-display"><span class="katex"><span class="katex-mathml"><math><semantics><mrow><msubsup><mo>∫</mo><mrow><mo>−</mo><mi>∞</mi></mrow><mi>∞</mi></msubsup><msup><mi>e</mi><mrow><mo>−</mo><msup><mi>x</mi><mn>2</mn></msup></mrow></msup><mi>d</mi><mi>x</mi><mo>=</mo><msqrt><mi>π</mi></msqrt></mrow><annotation>\int_{-\infty}^{\infty} e^{-x^2} dx = \sqrt{\pi}</annotation></semantics></math></span><span class="katex-html"><span class="base"><span class="strut" style="height: 2.3846em; vertical-align: -0.9703em"></span><span class="mop"><span class="mop op-symbol large-op" style="position: relative; top: -0.0011em">∫</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height: 1.4143em"><span style="top: -1.7881em"><span class="pstrut" style="height: 2.7em"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mtight">−</span><span class="mord mtight">∞</span></span></span></span><span style="top: -3.8129em"><span class="pstrut" style="height: 2.7em"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mtight">∞</span></span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height: 0.9703em"><span></span></span></span></span></span></span><span class="mspace"></span><span class="mord"><span class="mord mathnormal">e</span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height: 1.0369em"><span style="top: -3.113em"><span class="pstrut" style="height: 2.7em"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mtight">−</span><span class="mord mtight"><span class="mord mathnormal mtight">x</span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height: 0.8913em"><span style="top: -2.931em"><span class="pstrut" style="height: 2.5em"></span><span class="sizing reset-size3 size1 mtight"><span class="mord mtight">2</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span><span class="mord mathnormal">d</span><span class="mord mathnormal">x</span><span class="mspace"></span><span class="mrel">=</span><span class="mspace"></span></span><span class="base"><span class="strut" style="height: 1.04em; vertical-align: -0.1908em"></span><span class="mord sqrt"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height: 0.8492em"><span class="svg-align" style="top: -3em"><span class="pstrut" style="height: 3em"></span><span class="mord"><span class="mord mathnormal">π</span></span></span><span style="top: -2.8092em"><span class="pstrut" style="height: 3em"></span><span class="hide-tail" style="height: 1.08em"><svg width="400em" height="1.08em"><path></path></svg></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height: 0.1908em"><span></span></span></span></span></span></span></span></span></span>

src/vs/workbench/contrib/markdown/test/browser/markdownKatexSupport.test.ts

@@ -35,5 +35,15 @@ suite('Markdown Katex Support Test', () => {
 		const rendered = await renderMarkdownWithKatex('Hello ($\\frac{1}{2}$) World!');
 		await assertSnapshot(rendered.element.innerHTML);
 	});
+
+	test('Should support blocks immediately after paragraph', async () => {
+		const rendered = await renderMarkdownWithKatex([
+			'Block example:',
+			'$$',
+			'\\int_{-\\infty}^{\\infty} e^{-x^2} dx = \\sqrt{\\pi}',
+			'$$',
+		].join('\n'));
+		await assertSnapshot(rendered.element.innerHTML);
+	});
 });