I’ve added this feature as part of my update to the AlphaTrack intergration tool, but I think it would be useful to make it native.
Current behaviour is that when changing the pattern length, left mouse button changes by one, while right mouse button changes by 10 (same as all other controls of this type).
Suggested addition to this is if a modifier is held (Ctrl/Alt/etc.) then left mouse button would change the pattern length to an even multiple of LPB (effectively beats).
For example, if LPB is 4 and the pattern length was 16, changing with the modifier would set the new length to 8, 12, 16, 20, 24 etc.
If the original length was not an even number of LPB, round this with the first change (i.e. LPB=4 and pattern length = 14 would result in a value of either 12 or 16 depending on the direction of the mouse).
Hope this is clear.
NB: I may release a tool that adds keybinding for this purpose, but will cannot integrate the mouse behaviour.