The Composition viewer ignored drags from other panels. Now, as in After
Effects:
- A Project item dropped on the viewer becomes a layer centred where it
lands (above the selected layer, like any new layer); while dragging, the
viewer is outlined and the layer's frame follows the pointer.
- Media Browser files do the same; files dropped from the OS file manager
do too when the platform reports where the pointer is during the drag
(otherwise they import as before).
- An effect from Effects & Presets goes on the layer under the pointer,
which is outlined while dragging.
layer.addItem and layer.newModel take `position` ([x, y] comp px).
file.import takes `addToComp` with `time`, `index` and `position`;
mediaBrowser.import shares it (file::add_to_comp): layered files go in as
their comps, the comp they were dropped in stays open, and items that
can't be layers are reported in `errors`. viewer::layer_at is the hit
test pick() and the drop share.
Tests: ui_drag_drop.rs (project item and effect dropped on the viewer)
and imported_files_go_into_the_comp_where_they_were_dropped (engine).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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