fix(selection): drag-scroll end row and trim start clamp (#5993)

* fix(selection): drag-scroll end row matches last visible line

Use ydisp + rows - 1 for selectionEnd when scrolling down during drag,
consistent with finalSelectionEnd for select-all.

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

* fix(selection): reset start to origin when start row is trimmed

When handleTrim removes the selection start row, clamp to [0,0] and
refresh; do not keep a stale column on row 0.

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

---------

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

src/browser/selection/SelectionModel.test.ts

@@ -64,6 +64,13 @@ describe('SelectionModel', () => {
       assert.deepEqual(model.finalSelectionStart, undefined);
       assert.deepEqual(model.finalSelectionEnd, undefined);
     });
+    it('should reset selection start to origin when start row is trimmed', () => {
+      model.selectionStart = [50, 0];
+      model.selectionEnd = [10, 2];
+      assert.equal(model.handleTrim(1), true);
+      assert.deepEqual(model.finalSelectionStart, [0, 0]);
+      assert.deepEqual(model.finalSelectionEnd, [10, 1]);
+    });
   });
 
   describe('finalSelectionStart', () => {