There is a bug in SMath GUI when working in mono (Linux version). When you select an expression with mouse, the selection box displayed is too narrow if the font size is larger than default (vertical box size is OK). But the real issue is that number's horizontal sizes are incorrect too that leads in buggy editing and appearance of numbers/expressions.
Steps to reproduse:
1. Use Linux SMath version. I use now SMath 0.99.6884 witn mono 4.6.2 (yum install mono-winforms) in CentOS 7.5 x64.
2. Select, for example, 14th font (10 is too small). It's important to increase the font relative to default or the bug will not appear.
2. Type something like 2500*10^-3 <Enter>. You will see that the multiplication sign is inside the number area. Now try to delete digits. You'll see that the cursor is drawn at incorrect position. Something in the code is not aware that the user have changed the font size, so default horizontal size is used in some places of the code.