I think the easiest way forward is to double-click the value, take my hand off the mouse so it doesn't get nudged, and overtype the value. The issue is that the cursor is in the way. It also means I have to replace the value in its entirety when all I might want to do is change a numeral.
I did a few more tests...
If I double-click the value and immediately drag the cursor down to the right, then take my hand of the mouse so as not to nudge it, the field remains in focus. Any further movement of the mouse causes the field to lose focus.
If I double-click the value and immediately drag down to the left, the field loses focus immediately.
I also have an old 2012 MacBook Pro that I haven't used for a while, so I'll have a look to see if I have the same problems there.
I did a few more tests...
If I double-click the value and immediately drag the cursor down to the right, then take my hand of the mouse so as not to nudge it, the field remains in focus. Any further movement of the mouse causes the field to lose focus.
If I double-click the value and immediately drag down to the left, the field loses focus immediately.
I also have an old 2012 MacBook Pro that I haven't used for a while, so I'll have a look to see if I have the same problems there.