Fixes for drag and drop at the end of the document (#1039)

* Fix for DND highlight when no line break at EOF.

Closes #823

* Analogous fix for end of document indent level.

Author: microbit-matt-hillsdon

src/editor/codemirror/dnd-decorations.test.ts

@@ -131,6 +131,34 @@ describe("dndDecorations", () => {
 
     expect(plugin.decorations.size).toEqual(0);
   });
+
+  it("handles inserting at end of doc with no blank line", () => {
+    const view = createView(Text.of(["#1"]));
+    const plugin = new DndDecorationsViewPlugin(view, 0);
+
+    const viewUpdate = createViewUpdate(
+      view,
+      true,
+      view.state.update({
+        userEvent: "dnd.preview",
+        changes: [
+          {
+            insert: "\n#2\n",
+            from: 2,
+          },
+        ],
+      })
+    );
+    plugin.update(viewUpdate);
+    expect(viewUpdate.state.doc.sliceString(0)).toEqual("#1\n#2\n");
+
+    expect(decorationDetails(plugin)).toEqual([
+      {
+        class: "cm-preview",
+        from: 3,
+      },
+    ]);
+  });
 });
 
 interface DecorationDetails {

src/editor/codemirror/dnd-decorations.ts

@@ -35,7 +35,12 @@ export class DndDecorationsViewPlugin {
       const isDrop = transactions.some((t) => t.isUserEvent("dnd.drop"));
       if (isPreview || isDrop) {
         changes.iterChangedRanges((_fromA, _toA, fromB, toB) => {
-          const start = state.doc.lineAt(fromB);
+          let start = state.doc.lineAt(fromB);
+          // If there was no blank line at the end then the insertion will have
+          // started from the EOL but doesn't make sense to highlight that line.
+          if (start.to === fromB) {
+            start = state.doc.lineAt(fromB + 1);
+          }
           const end = state.doc.lineAt(toB);
           for (let l = start.number; l < end.number; ++l) {
             const line = state.doc.line(l);

src/editor/codemirror/edits.test.ts

@@ -493,4 +493,13 @@ describe("edits", () => {
       indentLevelHint: 0,
     });
   });
+  it("uses correct preceding line for indent at end of document", () => {
+    check({
+      line: 2,
+      initial: "if True:",
+      additional: "pass",
+      expected: "if True:\n\tpass\n",
+      type: "example",
+    });
+  });
 });

src/editor/codemirror/edits.ts

@@ -201,7 +201,12 @@ const indentLevel = (text: string): number => {
 };
 
 const preceedingLinesExclusive = function* (state: EditorState, from: number) {
-  const initial = state.doc.lineAt(from).number - 1;
+  const initialLine = state.doc.lineAt(from);
+  // Special case: if there was no line break on the previous line then
+  // we're inserting one. So we need to use the current line as the preceding
+  // line.
+  let initial =
+    initialLine.to === from ? initialLine.number : initialLine.number - 1;
   for (let line = initial; line >= 1; --line) {
     yield state.doc.line(line);
   }