Improve "Compare Text in Visible Editors" error message

Clarify that users need a split view with exactly 2 editors open,
and show how many editors are currently visible. Also update README
to remove the misleading "(i.e. tabs)" wording.

Relates to #35, #76

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ryu1kn

README.md

@@ -6,7 +6,7 @@
 * Multi cursor text selection.
 * User defined text normalization rules to reduce the noise in the diff (e.g. replace tab characters to spaces).
 * User defined text normalization rules can be toggled off without removing them from the configuration.
-* Compare text in 2 visible editors (i.e. tabs) with one action.
+* Compare text in 2 visible editors in a split view with one action.
 
 ![Compare two text selections](https://raw.githubusercontent.com/ryu1kn/vscode-partial-diff/main/images/public.gif)
 
@@ -35,7 +35,7 @@ A couple of requests from me when you raise an github issue.
 
 * `Compare Text in Visible Editors` (**Command ID:** `extension.partialDiff.diffVisibleEditors`)
 
-    Compares text in 2 visible editors.
+    Compares text in 2 visible editors. Split your editor to show exactly 2 files before running this command.
 
 * `Toggle Pre-Comparison Text Normalization Rules` (**Command ID:** `extension.partialDiff.togglePreComparisonTextNormalizationRules`)
 

src/lib/commands/compare-visible-editors.ts

@@ -13,7 +13,9 @@ export default class CompareVisibleEditorsCommand implements Command {
     async execute() {
         const editors = this.windowAdaptor.visibleTextEditors;
         if (editors.length !== 2) {
-            this.windowAdaptor.showInformationMessage('Please first open 2 documents to compare.');
+            this.windowAdaptor.showInformationMessage(
+                `This command requires exactly 2 visible editors, but ${editors.length} is/are currently open. Please split your editor to show 2 files (either horizontally or vertically) and try again.`
+            );
             return;
         }
 

src/test/lib/commands/compare-visible-editors.test.ts

@@ -60,11 +60,28 @@ suite('CompareVisibleEditorsCommand', () => {
         });
     });
 
-    test('it tells you that it needs 2 visible editors', async () => {
+    test('it tells you that it needs 2 visible editors when fewer are open', async () => {
         const {command, deps} = createCommand([editor1]);
         await command.execute();
 
-        verify(deps.windowAdaptor.showInformationMessage('Please first open 2 documents to compare.'));
+        verify(deps.windowAdaptor.showInformationMessage(
+            'This command requires exactly 2 visible editors, but 1 is/are currently open. Please split your editor to show 2 files (either horizontally or vertically) and try again.'
+        ));
+    });
+
+    test('it tells you that it needs 2 visible editors when more than 2 are open', async () => {
+        const editor3 = mockType<TextEditor>({
+            viewColumn: 3,
+            selectedText: 'SELECTED_TEXT_3',
+            fileName: 'FILE3',
+            selectedLineRanges: [{start: 25, end: 30}]
+        });
+        const {command, deps} = createCommand([editor1, editor2, editor3]);
+        await command.execute();
+
+        verify(deps.windowAdaptor.showInformationMessage(
+            'This command requires exactly 2 visible editors, but 3 is/are currently open. Please split your editor to show 2 files (either horizontally or vertically) and try again.'
+        ));
     });
 
     function createCommand(visibleTextEditors: TextEditor[]) {