@test without applying this patch, field "Test 2" is only visible when field "Test 1" is selected "Yes".
Tested on Joomla! 3.4.2-dev staging in the global configuration form.

@n9iels you mean that for you works good without this patch?
hm, strange

yes, for me it works good without this patch. Mebay someone else can test this PR to?

well, @n9iels you are right in global configuration it works,
but in other places do not works (template, module, menu xml)...
now I understand nothing, hm

ok, answer is simple, there the fields rendered in different way

please do test in other places: template, module or menu item configuration,
I updated the description

@test successful, now it works as expected :-) Tested in mod_custom

@test successful

p.s.
maybe could be out of scope of this pr
i was wondering about this scenario

<select name="mys">
    <option value="" disabled="disabled" selected="selected">Please select an option</option>
    <option value="1">One</option>
    <option value="2">Two</option>
      ....
      ....
    <option value="n">n-option</option>
</select>

in this situation could be usefull to trigger the showon feature when any of a "real" options is selected

the .xml syntax could be something like this:

showon="mys:*"

cause this syntax

showon="mys:1,2,...n"

could not be suitable if the parent select option values are dynamic (ie loaded from a query)

what do you think ?

@alikon for this need change the script that toggle visibility,
I think if allow to use RegExp (like showon="mys:(.*)") then it will be a lot more powerful,
but it a lot more complex task

using RegExp became too complex imho even if more powerfull,
i like power of simplicity

RTC Thanks for tests and coding here!