Switching modes clears the displayed value

When I switch between calculator modes, the entered values is cleared to zero.

**Steps to reproduce**
1. Press Alt+1 to enter simple mode
2. Type anything, say 123
3. Press Alt+3 to enter engineering mode

**Expected behavior**
It is expected that 123 is displayed, however you see 0 instead

**Additional context**
It is a longstanding bug. Please fix it. It is very annoying.
