In 9.2 Blizzard made a change that affected the tolerance in timing between a key push and a target switch. It's WAY tighter now than it used to be, and for those of us who have 15 years of reflexive muscle memory as to the timing of our heals when switching targets, it now often results in heals landing on the previous target rather than the new target.
After hours and hours of investigation, it was discovered the root cause of this issue is a change in the following line, introduced in 9.2:
CompactUnitFrame.lua line 270 switched from:
frame:RegisterForClicks(“LeftButtonDown”, “RightButtonUp”);
to:
frame:RegisterForClicks(“AnyUp”);
The implication of this is that you used to be able to select your new target on a left mouse button downclick, but this new system only registers the switch on the upclick. This adds a split second of new latency to the timing of the switch — just enough to totally break muscle memory.
Overwriting line 270 with this line totally fixes this issue:
frame:RegisterForClicks(“LeftButtonDown”, “AnyUp”);
Until Blizzard decides to add the above fix, or adress it in a different way, you can use this.
Just note, this fix only applies to the stock raid frames, not party frames or other unit frames.