Drop structure highlighting runs with a zero-width header

src/editor/codemirror/structure-highlighting/blocks.ts

@@ -34,6 +34,7 @@ export interface CodeBlock {
    * Start of the first header node of the run.
    */
   start: number;
+  /** Always past {@link start}, so `bodyStart - 1` is a valid position. */
   bodyStart: number;
   bodyEnd: number;
   /**
@@ -80,13 +81,19 @@ export const codeBlocks = (state: EditorState): CodeBlock[] => {
           let runStart = 0;
           for (let i = 0; i < children.length; ++i) {
             if (children[i].name === "Body") {
-              blocks.push({
-                statement: leaving.name,
-                start: children[runStart].start,
-                bodyStart: children[i].start,
-                bodyEnd: children[i].end,
-                depth,
-              });
+              // Error recovery can give a compound statement a zero-length
+              // error node in place of its header, leaving the body starting
+              // where the header does. There's no header to draw, and the
+              // view relies on bodyStart being past start.
+              if (children[i].start > children[runStart].start) {
+                blocks.push({
+                  statement: leaving.name,
+                  start: children[runStart].start,
+                  bodyStart: children[i].start,
+                  bodyEnd: children[i].end,
+                  depth,
+                });
+              }
               runStart = i + 1;
             }
           }

src/editor/codemirror/structure-highlighting/grammar.test.ts

@@ -150,6 +150,32 @@ describe("structure highlighting grammar contract", () => {
     ]);
   });
 
+  it("drops runs whose header is only an error node", () => {
+    // A leading colon recovers as a TryStatement whose children are a
+    // zero-length error node then the body, so start and bodyStart are both 0.
+    // view.ts would take bodyStart - 1 and ask for line -1.
+    const state = createState(":\n    pass\n");
+    expect(codeBlocks(state)).toEqual([]);
+  });
+
+  it("keeps bodyStart past start for every run", () => {
+    const samples = [
+      "if a:\n    pass\n",
+      "if a:\n    pass\nelif b:\n    pass\nelse:\n    pass\n",
+      "try:\n    pass\nexcept:\n    pass\nfinally:\n    pass\n",
+      "if a: pass\n",
+      ":\n    pass\n",
+      "::\n    pass\n",
+      "    pass\n",
+      "else:\n    pass\n",
+    ];
+    for (const doc of samples) {
+      for (const block of codeBlocks(createState(doc))) {
+        expect(block.bodyStart).toBeGreaterThan(block.start);
+      }
+    }
+  });
+
   it("covers every compound statement name in grammarInfo", () => {
     const samples = [
       "if a:\n    pass\n",

src/editor/codemirror/structure-highlighting/view.ts

@@ -79,7 +79,8 @@ export const codeStructureView = (option: "full" | "simple") =>
          *
          * @param view The view.
          * @param start The start position.
-         * @param end The end position.
+         * @param end The end position. Must be greater than zero, as we look
+         *            at the character before it; see CodeBlock.bodyStart.
          * @param depth Current indent depth (1 per indent level starting at 0).
          * @param parent The parent positions (e.g. for the while block) if we're calculating body positions, otherwise undefined.
          * @returns The positions for the block denoted by start/end or undefined if highlighting should be skipped.