fix(lynxtron-go): preserve loaded content on undo (#64)

Author: icecreamx10

.changeset/calm-tools-undo.md

@@ -0,0 +1,5 @@
+---
+"lynxtron-go": patch
+---
+
+Prevent Cmd+Z from clearing an untouched editor after Lynxtron Go loads a file.

lynxtron-go/scintilla-extension/module/scintilla_view.mm

@@ -480,6 +480,15 @@ static bool ScxVerbose() {
             if (current == text) return;
         }
         [container.scintillaView message:SCI_SETTEXT wParam:0 lParam:(sptr_t)text.c_str()];
+        // Host-driven replacement is a document load, not a user edit.
+        // SCI_SETTEXT records the inserted document in Scintilla's undo
+        // history, so the first Cmd+Z on an untouched editor otherwise
+        // removes the entire file. Also discard history from the previously
+        // displayed IDE tab: applying that history to this document would be
+        // equally incorrect. User edits made after this point are collected
+        // normally and remain undoable.
+        [container.scintillaView message:SCI_EMPTYUNDOBUFFER wParam:0 lParam:0];
+        [container.scintillaView message:SCI_SETSAVEPOINT wParam:0 lParam:0];
         [container.scintillaView setNeedsDisplay:YES];
     };
     if ([NSThread isMainThread]) {

lynxtron-go/src/app/ide/scintilla-undo.test.ts

@@ -0,0 +1,28 @@
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { describe, expect, it } from 'vitest';
+
+const TEST_DIR = path.dirname(fileURLToPath(import.meta.url));
+const SCINTILLA_VIEW_SOURCE = path.resolve(
+  TEST_DIR,
+  '../../../scintilla-extension/module/scintilla_view.mm',
+);
+
+describe('Scintilla host content replacement', () => {
+  it('starts loaded documents with an empty native undo history', () => {
+    const source = fs.readFileSync(SCINTILLA_VIEW_SOURCE, 'utf8');
+    const setContent = source.slice(
+      source.indexOf('void ScintillaView::SetContent'),
+      source.indexOf('std::string ScintillaView::GetContent'),
+    );
+
+    const setText = setContent.indexOf('message:SCI_SETTEXT');
+    const emptyUndo = setContent.indexOf('message:SCI_EMPTYUNDOBUFFER', setText);
+    const savePoint = setContent.indexOf('message:SCI_SETSAVEPOINT', emptyUndo);
+
+    expect(setText).toBeGreaterThanOrEqual(0);
+    expect(emptyUndo).toBeGreaterThan(setText);
+    expect(savePoint).toBeGreaterThan(emptyUndo);
+  });
+});