When channels are scrolled to be (partially) behind the search bar,
their widget buttons would still be interactive, preventing the seach
buttons to be usable.

We have to make sure the events are consumed from the search and dont
reach other UI blocks.
We can do so by flagging the block UI_BLOCK_CLIP_EVENTS -- but also
have to make sure the bounds are calculated correctly (otherwise the
check relating UI_BLOCK_CLIP_EVENTS in ui_but_find_mouse_over_ex wont
trigger properly.