Hi,
I have tested your scenario and could not find the issue as you mentioned. When I double click (in edit mode) on a cell value (having double value), it selects all the value including before decimal and after decimal parts.
Could you create us a sample project, zip it and post it here to show the issue, also provide your template file here. We will check your issue soon.
Also, which browser type you are using, I have also tested on Google chrome it works fine.
But I have noticed on FireFox that if I place the mouse cursor at the start of the value (in edit mode), it only selects first part before decimal places, similarly, it selects the second part if I place the pointer just after the decimal. Do you confirm this issue, we will log it and try to figure it out for FireFox and other IE browser versions.
Thank you.