fix(editor): Allow selecting and copying text in read-only code previews (#136)

Code previews (Find in Files, Usages) are non-editable CodeMirror views, so their text inherited the workbench's user-select: none and had no tab stop: it couldn't be focused, selected with the keyboard or copied. Previews now opt their content back in with the shared selectable-text class and take a tab stop; editing stays blocked. Same root cause as the read-only diff panes (#126).

src/editor/core/cm/preview.test.ts

@@ -0,0 +1,18 @@
+import { describe, expect, it } from "vitest";
+import { EditorState } from "@codemirror/state";
+import { EditorView } from "@codemirror/view";
+import { selectableText } from "@/ui";
+import { previewExtensions } from "./preview";
+
+describe("previewExtensions", () => {
+  it("keeps the read-only preview selectable and focusable", async () => {
+    const view = new EditorView({
+      state: EditorState.create({ doc: "let a = 1;", extensions: await previewExtensions("/p/a.txt", undefined) }),
+    });
+    expect(view.state.readOnly).toBe(true);
+    expect(view.contentDOM.getAttribute("contenteditable")).toBe("false");
+    expect(view.contentDOM.getAttribute("tabindex")).toBe("0");
+    expect(view.contentDOM.classList.contains(selectableText)).toBe(true);
+    view.destroy();
+  });
+});

src/editor/core/cm/preview.ts

@@ -13,6 +13,7 @@ import { openideaTheme } from "./theme";
 import { findLanguageDescription, loadSupport, loadedSupport, uiModeForPath } from "./languages";
 import { fontExtension } from "./setup";
 import { tx } from "@/platform/i18n";
+import { selectableText } from "@/ui";
 
 export async function previewExtensions(path: string, settings: EditorSettings | undefined): Promise<Extension> {
   const open = editorsFor(path)[0]?.ctx;
@@ -26,7 +27,8 @@ export async function previewExtensions(path: string, settings: EditorSettings |
     syntaxHighlighting(openideaHighlightStyle),
     EditorState.readOnly.of(true),
     EditorView.editable.of(false),
-    EditorView.contentAttributes.of({ "aria-label": tx`Preview of ${path}` }),
+    // Not editable, so selection and Copy need the text opted back in and a tab stop.
+    EditorView.contentAttributes.of({ "aria-label": tx`Preview of ${path}`, class: selectableText, tabindex: "0" }),
     support ?? [],
   ];
 }