Fix scrolling in Blockly

[palas]

See feedback:

In some windows, attempting to move the mouse out of the window causes automatic scroll. Good! This is both intuitive and convenient. But in the window for composing a Blockly program, one needs to explicitly move the scroll bars to scroll the window, which is inconvenient. The automatic scroll behaviour would make it much easier to compose programs, since even the simplest program is likely to exceed the window size.