Datepicker: Fixed incorrect position on position:fixed elements

[staaky]

Datepicker offsets are not calculated the same across methods. This causes incorrect positioning when a datepicker is attached to a bordered element within a position:fixed container after the page is scrolled.

I've changed _showDatepicker to use outerHeight() when measuring the input, so it matches the calculation in _checkOffset. This way _checkOffset no longer tries to compare two differently calculated offsets.

[jsf-clabot]

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[chadrschroeder]

Thank you for making this change. We're also seeing this issue when the browser zoom isn't at 100%, the page is scrolled down, and then a fixed position modal form is displayed which contains a datepicker.

For example, when the zoom is at 100% we'll have something like:

// In _showDatepicker, input.offsetHeight is "36"
$.datepicker._pos[ 1 ] += input.offsetHeight; // add the height

// In _checkOffset, inst.input.outerHeight() is "36"
inputHeight = inst.input ? inst.input.outerHeight() : 0,

// In _checkOffset, this calculation works correctly and $(document).scrollTop() is subtracted
offset.top -= ( isFixed && offset.top === ( inst.input.offset().top + inputHeight ) ) ? $( document ).scrollTop() : 0;

When the zoom is at 80% we'll have something like:

// In _showDatepicker, input.offsetHeight is "38"
$.datepicker._pos[ 1 ] += input.offsetHeight; // add the height

// In _checkOffset, inst.input.outerHeight() is "37.7344"
inputHeight = inst.input ? inst.input.outerHeight() : 0,

// In _checkOffset, this calculation doesn't work correctly and the datepicker is positioned too far down on the page
offset.top -= ( isFixed && offset.top === ( inst.input.offset().top + inputHeight ) ) ? $( document ).scrollTop() : 0;