Structure highlighting bug fixes (#1061)

* Don't ask for line information past EOF.

Closes #1060

Can be reproduced with `if True:pass` at EOF and, importantly, outside
the viewport (so prepended with blank lines, for example).

I'm not sure our behaviour in this scenario makes sense but this change
at least makes it safe.

* Detect the `if True:pass` case consistently.

Use a simpler approach to test if we're the same line to make it easier
to follow.

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

@@ -86,23 +86,36 @@ export const codeStructureView = (option: "full" | "simple") =>
 
         let cursorFound = false;
 
+        /**
+         * Calculate visual positions for a node from start/end.
+         *
+         * @param view The view.
+         * @param start The start position.
+         * @param end The end position.
+         * @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.
+         */
         const positionsForNode = (
           view: EditorView,
           start: number,
           end: number,
           depth: number,
-          body: boolean
+          parent: Positions | undefined
         ): Positions | undefined => {
           const diagnostics = state.field(lintState, false)?.diagnostics;
           const indentWidth =
             state.facet(indentUnit).length * view.defaultCharacterWidth;
-
           let topLine = view.lineBlockAt(start);
-          if (body) {
-            topLine = view.lineBlockAt(topLine.to + 1);
-            if (topLine.from > end) {
-              // If we've fallen out of the scope of the body then the statement is all on
-              // one line, e.g. "if True: pass". Avoid highlighting for now.
+          if (parent) {
+            if (topLine.to + 1 < view.state.doc.length) {
+              topLine = view.lineBlockAt(topLine.to + 1);
+            } else {
+              // There's no next line.
+              return undefined;
+            }
+            if (parent.top === topLine.top) {
+              // It's the same line, e.g. if True: pass
               return undefined;
             }
           }
@@ -179,14 +192,14 @@ export const codeStructureView = (option: "full" | "simple") =>
                       startNode.start,
                       bodyNode.start,
                       depth,
-                      false
+                      undefined
                     );
                     const bodyPositions = positionsForNode(
                       view,
                       bodyNode.start,
                       bodyNode.end,
                       depth + 1,
-                      true
+                      parentPositions
                     );
                     if (parentPositions && bodyPositions) {
                       blocks.push(