Steps to reproduce the issue

1. Create a number form field in a view form xml with the following minimum settings:

   name="mynumber" 
   type="number" 
   default="0" 
   min="-123" 
   

2. Create a new item using the above form data
3. The form field shows a value of -123 (wrong)
4. Then set the value of the 'mynumber' field to 0 (zero) and save
5. The form field will again show the min-value of -123 (wrong)

Expected result

Correct behavior when using negative values

Actual result

If the value is set to <= 0 the field value is set to its min-value.
Also the default value is ignored and the field's min-value is used instead.

System information (as much as possible)

Joomla 3.5, PHP 5.6

Additional comments

I did some research on the form field code and it seems that the "empty value detection" is not working as expected. Since empty(0) will return true, the specified min-value will always be used. This is okay as long as you use positive numbers, but fails with negative.

Here's a suggestion on fixing this (File: libraries/joomla/form/fields/number.php:160)

replace

$value = (float) $this->value;
$value = empty($value) ? $this->min : $value;

with

// Make sure to set a valid default value (use min if 'default' not set or 0 if 'default' and 'min' not set)
if($this->value === "") {
     if(isset($this->min)) {
          $value = (float) $this->min;
     } else {
          $value = 0;
     }
} else {
     $value = (float) $this->value;
}