Fix Media field lightbox in subform

[ciar4n]

Pull Request for Issue #11755

Summary of Changes

This PR fixes a media field lightbox issue when additional subforms are added extending the page height. This results in the lightbox background no longer filling the page. Setting the lightbox background div (#sbox-overlay) to 'fixed' ensures the background remains within the viewport regardless of the page height.

Testing Instructions

Create ../modules/mod_login/text.xml to which add the following...

<?xml version="1.0" encoding="UTF-8"?>
<form>
    <fieldset name="images" class="image-subform">
        <field name="image" type="media" default="" label="Label1"  description="Desc1" size="30" />
        <field name="text" type="textarea" filter="raw" rows="10" cols="42" default="" label="Label2" description="Desc2" />
    </fieldset>
</form>

Add the following to the mod_login.xml file:

<field name="subformtest" type="subform" formsource="modules/mod_login/text.xml" class="advancedSelect" min="1" max="50" multiple="true" layout="joomla.form.field.subform.repeatable" groupByFieldset="true" label="Image(s)" description="Add image(s) to display in gallery" />

Then in the Module Manager, open the Login module. Add a number of subforms extending the page height. Click 'Select' in the last subform.

Before patch

After Patch

[C-Lodder]

Can't find this on JIssues, so marking this test as successful here

[zero-24]

Just reported it to jissues @C-Lodder joomla/jissues#871

[jeckodevelopment]

I have tested this item ✅ successfully on 8843ffd

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[zero-24]

@C-Lodder i have just added you test

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_{This comment was created with the J!Tracker Application at issues.joomla.org/joomla-cms/11760.}

[zero-24]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/joomla-cms/11760.}

[jeckodevelopment]

bot is sleeping!

[mbabker]

No it's not. We're fighting with ModSecurity yet again.