Currently, successfully copy-pasting Keyframes from one Timeline to another, requires the desired Destination Element to first be selected before the Paste of the Keyframes can be successfully completed.

However, we offer no feedback pertaining to this mechanic, making it appear as if the function is generally not available ( https://www.noesisengine.com/bugs/view.php?id=5042 ).

In the similar situation of attempting to Paste an Element into an already-populated single-child Element, we flash a Yellow Toast Notification stating:
"Can't paste here"
"The element you're trying to paste into can have can't have more than one child"

We should consider doing the same when attempting to Paste into a Timeline without an valid Element selected first:

"Can't Paste here"
"An element needs to first be selected to paste keyframes into it"

Repro Steps:

1) Create a Studio Project featuring a MainPage.xaml

2) Navigate to the Animation/Storyboard Tab, and create a new Storyboard, and create a few Keyframes for any Element along the Timeline.

3) Select all of the frames created in Step 2), and press [Ctrl+C] to copy them.

3) Create a new Storyboard, enter it, and without selecting any Element, press [Ctrl+V].
Observe how no Paste operation has taken place.
Observe how no indication is given as to why the Paste has not been able to take place.

Expected Results: A toast is provided to the user as to the reason for the operation not functioning as requested.