UPDATE: Since people clamored for reversion, here are a couple ideas on how Avid could do that while still allowing on-the-fly snap to grid within slip mode: Hit control *after* clicking a breakpoint. (This currently does nothing, whether editing a midi note, clip or breakpoint.) Or make command or some other mod key or combo toggle slip/grid only in slip-grid combo mode (which is apparently identical to slip mode).
There are more flaws and inconsistencies in breakpoint editing than this character limit allows. I hope Avid takes a more thorough look.
In 12.6, 12.5.2, and 12.4, horizontal-only automation breakpoint adjustment is broken.
In 12.6, shift-option-command or shift-option or command-option and then click/dragging an automation breakpoint = horizontal only movement, but the breakpoint snaps back to where you started when releasing the click and keys (in any order). Click/dragging first and then hitting shift-option-command = unrestricted movement in all directions (with vertical fine adjustment). I.e., shift-option-command is either horizontal only or unrestricted depending on whether you click before or after modifier keys, and horizontal only movement is bugged.
Slip-grid toggle with these mod key combos is also inconsistent. I don't have space to enumerate all the inconsistencies.
Horizontal-only movement does work on clip gain line, though hitting shift-option-control after clicking a breakpoint is still horizontal-only movement (whereas it's unrestricted fine movement when adjusting an automation breakpoint).
In 12.4 & 12.5.2, horizontal-only movement of automation breakpoints with the smart tool is entirely impossible. Horizontal-only movement of clip gain breakpoints is doable.
In all 3 versions, shift-option-click and command-option-click with the grabber tool add breakpoints in line with previous or subsequent breakpoints. With the smart tool, these mod key combos do nothing.
In 12.6, horizontal only movement does put the automation breakpoint where you drag it. But implementation is sometimes opposite of what you'd expect given that command is supposed to toggle slip/grid. For example:
In grid mode, both shift-option and command-option then click/dragging = smooth horizontal only movement.
shift-option-command = horizontal only movement that snaps to grids
These command key combos also behave differently if hit *after* clicking: shift-option and command-option = unrestricted smooth movement; shift-option-command = unrestricted fine movement that snaps to grid.
Note that unrestricted fine breakpoint adjustment in grid mode is impossible in 12.6.
The fact that certain mod key combos behave differently if hit before or after clicking a breakpoint is not a flaw in and of itself. I see it as an opportunity, along with the above mod key redundancies, for Avid to expand functionality (like on-the-fly slip/grid toggle) without crippling existing functions.