How to reproduce :

• on the input form element apply this styles :

  border: 0 none; box-sizing: border-box;

• Edit the form, select the input form element, and apply the Width to 100px.
• clic anywhere else and select again the input form element, the width value is 98px,
• clic anywhere else and select again the input form element, the width value is 96px,
• clic anywhere else and select again the input form element, the width value is 94px ... etc

The problem comes form line 1295 of /plugins/forms/resources/js/Ametys/plugins/forms/Components.js

var width = Ext.get(input).getWidth() - 2;

The correction :

var width = Ext.get(input).getWidth(true);

Be carreful that the same thing does not occurs with the height value.