This is a simple bug that is persistant in different SMath, mono and Linux version combinations. When you switch to other appliation, then back to SMath (for example, with Alt+Tab), the focus is not set to the placeholder, although cursor is displayed in right position.
Steps to reproduce:
1. Use Linux version of SMath. I use now SMath 0.99.6884, mono 4.6.2, CentOS 7.5. The issue is likely to appear in any combination.
2. Launch SMath.
3. Switch to other window with Alt+Tab.
4. Switch back to SMath. Type something. Nothing happens at cursor position.
Although it's a simple bug, it's very annoying when you copying numbers from another program/document.