[yordan-st]

Summary

• Adds Atlas Core styling for the new keyboard resize grip (.mx-resizer.mx-resizer-grip) and its keyboard hint (.mx-resizer-hint*) in resizable popups.
• Adds an Unreleased entry to the Atlas Core changelog.

Context

The Mendix client renders the grip and the hint as part of making popup resize controls accessible: WC-3495, with the design from CD-1007. Runtime change: https://gitlab.rnd.mendix.com/appdev/appdev/-/merge_requests/38848

The two changes can be merged in either order:

• Without this PR, the grip still works from the keyboard and shows a focus outline, but it is invisible, like the current resize handles.
• Without the runtime change, these selectors match nothing.

Details

• Grip: overrides the legacy .mx-resizer-se position (20×20 px, inset 2 px from the corner). It shows a dot icon drawn with a CSS mask in currentColor, which changes to --brand-primary on hover and focus.
• Hint: positioned absolutely below the popup, laid out as a three-column grid. It is hidden through [hidden] (toggled by the client), and its kbd styling overrides Bootstrap's.
• Uses only existing CSS variables with SCSS fallbacks; no new variables.

Test plan

• main.scss compiles with sass (only the existing warnings in _flex.scss)
• Visual check in a Studio Pro test app running the client from the appdev branch
• Check whether the e2e popup screenshots change