The pick-whip

Both surfaces already had a dropdown, and the dropdown stays: it is the right control when the layer you want is scrolled out of sight. The whip is the one people reach for first, and its absence is the thing that reads as "this is not really After Effects" more than any missing effect does.

Parenting#

Drag the spiral onto a layer in the scene tree or the timeline. The line turns solid and accented over a layer it will accept, and stays dashed over one it will not — a layer cannot parent to itself or to its own descendant, because that is a cycle, and the whip asks eligibleParents the same question the dropdown's list answers.

Parenting never moves the layer on screen. Both controls go through reparentNode, so it cannot mean two different things depending on which one you used.

Alt: the "jump" variant#

Hold Alt (Option) while dropping the whip — or while choosing from the parent dropdown — to link the layer without compensating its transform. Its values stay exactly as typed, so it jumps into the parent's coordinate space instead of staying put. After Effects has the same modifier.

Use it when the child's numbers are already authored relative to the parent — building a rig from measured offsets, or re-attaching something you positioned in parent space — where the default compensation is something you would undo by hand a second later. On an animated layer the difference is visible in the keyframes: the default offsets every one of them to hold the pose, Alt leaves them untouched.

All four surfaces (the inspector's picker, the compositing panel's, the timeline's Parent & Link column, and the whip on each) turn the modifier into an option through the one parentOptionsFor helper, so it cannot come to mean different things in different places.

Expressions#

Typing layer('Hero', 'y') requires knowing the function, the layer's exact name, and the property's internal key. Dragging requires knowing which layer you meant.

• Dropped on a property row, it references that property.
• Dropped on a layer, it references the same property this expression is on — "follow that layer's Y", which is what the gesture means when you drag from Y, and what After Effects produces for it.

The reference is inserted at the caret and the caret lands after it, so you can keep typing. A layer name containing a quote is escaped rather than emitted as broken syntax. The whip refuses its own property: an expression that reads itself is a cycle the evaluator would refuse anyway, and refusing at the gesture is a better place to say so.

How a surface becomes a drop target#

Targets are attributes, not a registry:

data-whip-layer="<nodeId>"    <!-- this element is a layer -->
data-whip-prop="<prop>"       <!-- …and this property of it -->
data-whip-scope="layer"       <!-- inside here, data-id is a layer id -->

The obvious alternative — a store every droppable row registers its bounds with — is the wrong one here. Rows are virtualised in both the scene tree and the timeline, they move on every scroll, and a registry of live rectangles would need invalidating by scroll, resize, panel drag, zoom and layout change: five subscriptions to maintain a copy of something the browser already knows exactly. So hit-testing is elementFromPoint, and adding a target is one attribute.

The scope form exists for the scene tree, whose rows already carry data-id from the shared TreeView. Marking the container is one line; teaching TreeView to emit whip attributes would be a new API on a component six panels use with four different kinds of id.

Implementation notes#

• Pointer events, not HTML drag-and-drop. Native DnD cannot draw a line, puts its own drag image in the way, and gives no pointer position during a drag on Firefox.
• Pointer capture is what makes the drag survive leaving the button. Without it the events go to whatever is under the cursor and the line stops following after ten pixels.
• The line is a portal on document.body. The whip lives inside panels that clip and scroll, and a line drawn in place would be cut off at the first panel edge it crossed — which is every drag that matters.