Use a minimal size of 2 for the select tag of MultiSelection widgets

Browsers based on Chromium renders the select as a "normal" select tag when
size=1. Such that the mousedown event is triggered twice which clear the
selection.

Closes #14529
(grafted from 4d95b908a33d950d24416b53d6264db1b0b3b443)

--HG--
branch : 7.0

sao/src/view/form.js

@@ -4471,7 +4471,9 @@ function eval_pyson(value){
                     yexpand = this.expand;
                 }
                 if (!yexpand) {
-                    this.select.prop('size', this.select.children().length);
+                    this.select.prop(
+                        'size', Math.max(2, this.select.children().length));
+
                 }
                 if (!field) {
                     return;