[FIX] web pivot: Fix open position of selection

Description of the issue/feature this PR addresses:
In pivot, when we click on '+', it must open the selection box at
the point where we click.

Current behavior before PR:
The box opens at the top of the page

Desired behavior after PR is merged:
The box opens at the point where we click

id=2066675

closes odoo/odoo#37385

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>

addons/web/static/src/scss/pivot_view.scss

@@ -91,6 +91,10 @@
         }
     }
 
+    .o_field_selection {
+        position: absolute;
+    }
+
     .o_pivot_field_selection::after {
         @include o-caret-right;
         position: absolute;