Currently, the hitboxes on keyframes when dragging-and-selecting with the marquee are super unforgiving, bordering into imprecise.

The best demonstration of this is the attached video.
In it, can be seen that even when extending the marquee beyond the geometry of the diamond shape, the first keyframe is still not selected.
It takes extending the marquee far further outside of frame to correctly make the selection of the first keyframe.

Repro Steps:

1) Create a Studio Project featuring a MainPage.xaml

2) Create a Storyboard in the Animation/Storyboard Panel

3) On the first frame, key any Property change.

4) By clicking and dragging on the Timeline (to generate the 'selection' marquee), drag the edge of the marquee over the right edge of the Keyframe diamond (without passing the center)

Observe how the first keyframe is not selected.

5) By clicking and dragging on the Timeline (to generate the 'selection' marquee), drag the edge of the marquee over the center of the Keyframe diamond.

Observe how the first keyframe is not selected.

6) By clicking and dragging on the Timeline (to generate the 'selection' marquee), drag the edge of the marquee until touching the left edge of the Keyframe diamond.

Observe how the first keyframe is not selected.

Expected Results: The marquee being positioned over any part of the Keyframe diamond shape, should engage a selection of it.