This is not a bug. It is per design and in order to make it possible for user to insert into the BLOB-editor without using the mouse. A favour we did users that prefer not to use mouse.
SQLyog 8.05 release notes:
“In DATA and RESULT tab you can now open the BLOB VIEWER for BLOB and TEXT types with any keyboard keystroke (like SPACEBAR). The ‘NOT NULL’ checkbox will then automatically uncheck (if checked) and BLOB VIEWER window is ready for input from the keyboard without any mouse action”