A Project item (or Media Browser file) dropped in the Timeline always
went above the selected layer, starting where new layers start.
Now, as in After Effects, a line shows where it goes in the stack (above
the first layer row whose middle is below the pointer, else below the
last), and over the time graph a marker with a timecode tag shows its In
point, which follows the pointer (frame-snapped) or, with Shift, the
current time. The layer reorder drag shares the insertion logic.
layer.addItem and layer.newModel take `index` (1-based stack position);
mediaBrowser.import passes `time` and `index` on, one layer under the
other.
Tests: project_items_land_where_they_are_dropped_in_the_timeline
(ui_drag_drop.rs, real pointer drags: outline, time graph, Shift, below
the last layer) and add_item_at_a_stack_index_and_time (engine).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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