Is it any part of a transaction? What kind of register are you looking at? Bank, investment etc. There was a problem a long time ago with wireless mice and a particular version of Java but that shouldn't be an issue with a new Mac and a new register.
Have you set any options on font size in the preferences? Or different fonts for a register?
It is not the number of entries, I have accounts going back to 1998 or so.
I have a number of accounts which are just for my own personal finances so a very simple setup on each one. I tried to alter the setting to try and increase the size of font on the transaction page as I have rather poor sight. This is not a major problem and I was just curious to know if there was any straightforward answer to this phenomenon particularly as it has occurred on two machines. I would add that it can happen on each of the accounts but does not occur all the time so just a wee bit frustrating. To explain the problem when you try to select a transaction in order to edit it as you place the mouse curser on the transaction it will suddenly bounce up and down, not a great amount, but enough to make you miss your target and select the wrong transaction. I will take a look at the settings and see if there are any adjustments that will prevent this.