When you move Keyframe-point out of the bound of Time Frame visible field - the Scrollbar in "Dope sheet" doesn't react
or the horizontal scrollBar behaves differently in "Dope sheet" and "Curves" tabs

2) How can we reproduce it using the example you attached
1 Create new project or use attached
2 Create cube
3 Open animation window and create new animation clip
4 Press "Add Curve" and add Cube:Position
5 Move Keyframe-point out of the bound of Time Frame visible field

Actual result: the Scrollbar not changes its size, but should do that

Please see attached video