Currently there is no way to do a "select all" in the Execute Anonymous window. Right-clicking and ctrl-A both do nothing. This is definitely needed for both the "Source to execute" and "Results" controls.

Also, please make the splitter between the "Source to execute" control and the "Results" control visible so it's easier to resize.

Last but not least, please fix the log level filtering in the "System Log" window in the main Salesforce app as that setting appears to be completely broken in all browsers.