win32 input mode: Avoid scrolling to bottom on modifier only

Fixes #5612

src/browser/CoreBrowserTerminal.ts

@@ -1156,9 +1156,10 @@ export class CoreBrowserTerminal extends CoreTerminal implements ITerminal {
       this.textarea!.value = '';
     }
 
+    const wasModifierOnly = this._keyboardService.useWin32InputMode && wasModifierKeyOnlyEvent(event);
     this._onKey.fire({ key: result.key, domEvent: event });
     this._showCursor();
-    this.coreService.triggerDataEvent(result.key, true);
+    this.coreService.triggerDataEvent(result.key, !wasModifierOnly);
 
     // Cancel events when not in screen reader mode so events don't get bubbled up and handled by
     // other listeners. When screen reader mode is enabled, we don't cancel them (unless ctrl or alt
@@ -1199,7 +1200,8 @@ export class CoreBrowserTerminal extends CoreTerminal implements ITerminal {
     // Handle key release for Kitty keyboard protocol
     const result = this._keyboardService.evaluateKeyUp(ev);
     if (result?.key) {
-      this.coreService.triggerDataEvent(result.key, true);
+      const wasModifierOnly = this._keyboardService.useWin32InputMode && wasModifierKeyOnlyEvent(ev);
+      this.coreService.triggerDataEvent(result.key, !wasModifierOnly);
     }
 
     this.updateCursorStyle(ev);
@@ -1410,5 +1412,10 @@ export class CoreBrowserTerminal extends CoreTerminal implements ITerminal {
 function wasModifierKeyOnlyEvent(ev: KeyboardEvent): boolean {
   return ev.keyCode === 16 || // Shift
     ev.keyCode === 17 || // Ctrl
-    ev.keyCode === 18; // Alt
+    ev.keyCode === 18 || // Alt
+    ev.keyCode === 91 || // Meta (Left)
+    ev.keyCode === 92 || // Meta (Right)
+    ev.keyCode === 93 || // Meta (Menu)
+    ev.keyCode === 224 || // Meta (Firefox)
+    ev.key === 'Meta';
 }

src/browser/Terminal.test.ts

@@ -401,6 +401,42 @@ describe('Terminal', () => {
       });
     });
 
+    describe('keyDown', () => {
+      it('should not scroll down on modifier-only input in win32 input mode', async () => {
+        term.options.vtExtensions = { win32InputMode: true };
+        term.coreService.decPrivateModes.win32InputMode = true;
+        (term as any).textarea = { value: '' };
+
+        await term.writeP('test\r\n'.repeat(term.rows * 3));
+        const startYDisp = term.buffer.ydisp;
+        term.scrollLines(-1);
+        const scrolledYDisp = term.buffer.ydisp;
+        assert.equal(scrolledYDisp, startYDisp - 1);
+
+        const evKeyDown = {
+          type: 'keydown',
+          key: 'Control',
+          keyCode: 17,
+          ctrlKey: true,
+          preventDefault: () => { },
+          stopPropagation: () => { }
+        } as KeyboardEvent;
+
+        const evKeyUp = {
+          type: 'keyup',
+          key: 'Control',
+          keyCode: 17,
+          preventDefault: () => { },
+          stopPropagation: () => { }
+        } as KeyboardEvent;
+
+        term.keyDown(evKeyDown);
+        assert.equal(term.buffer.ydisp, scrolledYDisp);
+        (term as any)._keyUp(evKeyUp);
+        assert.equal(term.buffer.ydisp, scrolledYDisp);
+      });
+    });
+
     describe('scroll() function', () => {
       describe('when scrollback > 0', () => {
         it('should create a new line and scroll', () => {