Whenever I used [Curve Editor] in 3ds Max, I always wondered why I couldn't easily vertically select all the keys at one frame. Now I have figured out the easy way. I should use [Track Bar] for it. Because [Track Bar] is away from [Curve Editor], they seemed to be so unrelated that I couldn't realize it, but the method seems to be official.
I'll show you the picture. Note those red rectangles.
As shown in the picture, if you select a collective key on [Track Bar] at the bottom, it gets highlighted and the corresponding keys in a pillar in [Curve Editor] in the separate floating window also get highlighted.
You can also move a pillar of keys in [Curve Editor] by grabbing and dragging the correspodning collective key on [Track Bar].
As a matter of fact, you don't need to use [Curve Editor] so as to deal with the all the key at one frame. You have only to use [Track Bar] for it.
