The bug:
Most text input fields are un-editable until the cursor hovers over it. This makes it disruptive to, say, open a command block and immediately begin typing.

To reproduce:
1. In creative mode, open your inventory
2. Begin typing letters – observe that nothing is happening
3. Hover over the "search" input box with your mouse, then hover off
4. Continue typing – observe that the field has started working

Affects:

• Creative menu
• Command blocks
• Anvils
• Writable books

Does not affect:

• Game chat
• Signs

More details:
After a field is selected, other fields can only be activated by clicking, not simply hovering. Essentially, the expected behavior is as follows:

• Default field immediately selected when GUI opened
• Select any field by clicking only, hovering alone should do nothing
• Clicking other buttons should deselect the selected field, with the exception of the creative menu, whose search bar should always be active if it's visible.