The composer model dropdown (#composerModelDropdown) is a child of
.composer-footer, which sets container-type:inline-size (and, under the Geist
Contrast skin, a backdrop-filter). Both establish a fixed containing block, so
the phone position:fixed dropdown resolved against the FOOTER (pinned to the
bottom of the screen) instead of the visual viewport — dropping the menu below
the fold on all skins (the "tapping the model picker does nothing / opens
off-screen" symptom in #6080).
Fix (Fable UX gate recommendation on PR #6105): on the phone path
(<=640px) reparent the dropdown to <body> and switch it to position:fixed via a
.model-dropdown--floating class, exactly the working #profileDropdown idiom, so
fixed positioning is viewport-relative on every skin. The visualViewport math
from PR #6105 is preserved (viewport-relative coordinates, above/below flip
preferring the larger space, width = viewport - 16px, titlebar-bottom clamp) and
so are the rAF-coalesced visualViewport resize/scroll listeners.
Desktop (>640px) is unchanged: closeModelDropdown() and the desktop branch of
_positionModelDropdown() restore the element into its original .composer-footer
slot and clear the phone inline styles, so the desktop popover stays an
absolutely positioned footer child, byte-for-byte identical to master. Because
the menu now leaves the footer on phones, no per-skin backdrop-filter override
is needed.
Adds tests/test_issue6080_model_picker_reparent.py, including a node-executed
DOM-parentage guard that runs _positionModelDropdown()/closeModelDropdown()
against a stub DOM and asserts the open phone dropdown has NO .composer-footer
ancestor (escaping the container-type trap) and is restored into the footer on
close and on desktop.
Co-authored-by: webtecnica <75556242+webtecnica@users.noreply.github.com>
