The bug
The text fields for the customized world generation (page four) do not enable repeated events for the keyboard. What this means is that you cannot hold down for example backspace and remove the complete input. Instead you have to press a key every time you want to activate it. As far as I know this is the only GUI where it is not explicitly enabled (based on MCP 1.8).
Code analysis
Code analysis by @unknown can be found in this comment.
Linked issues
is blocked by 1
relates to 2
Comments 7
Based on MCP 9.40pre-1, repeated events can be enabled in this GUI by adding Keyboard.enableRepeatEvents(true); to the net.minecraft.client.gui.GuiCustomizeWorldScreen.initGui() method. Also, repeated events need to be deactivated in the net.minecraft.client.gui.GuiCustomizeWorldScreen.onGuiClose() method (which isn't yet implemented).
Even though this is a pretty simple and obvious fix, it might be worth adding it to the ticket's description.
Can confirm for 1.10.2.
Related to MC-94502?