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.ts

@@ -135,9 +135,10 @@ export class SelectionModel {
       return true;
     }
 
-    // If the selection start is trimmed, ensure the start column is 0.
+    // If the selection start row is trimmed away, reset to the buffer origin.
     if (this.selectionStart && this.selectionStart[1] < 0) {
-      this.selectionStart[1] = 0;
+      this.selectionStart = [0, 0];
+      return true;
     }
     return false;
   }