Things I tested

• Focusing search opens the search modal and keeps focus on the input. Selection in the bottom list is cleared.
• The modal does not overflow the bottom of the screen, but scrolls internally
• Resizing the grid panel also updates the search modal size/position
• Clicking outside of the modal (except a resize handle and the input) closes the modal
• Pressing escape closes the modal
  • When input is focused
  • When modal is focused (Ctrl+Click an item)
• Clicking on the input or typing re-opens the modal
• Searching updates the matching items
• Clicking an item in the modal closes the modal, selects and scrolls to the item in the main list
• If there are multiple matching items, a "Select Matching" button appears as a footer (always visible). It selects all matching items and closes the modal.
• Ctrl+Click and Shift+Click work in the search modal. Note you must hold ctrl/shift for the first item click
  • When there are multiple items selected, a "Select Group" button appears in addition to the "Select Matching" button. This closes the modal (the items are already selected in the underlying list)
• You can drag an item out of the search modal and move it around the underlying list. You can drop in the list.
• You can Ctrl/Shift+Click multiple items in the search modal, then drag any of the selected items to move the entire selection
• Groups appear flattened in the modal
• If you disable "Show hidden items", those items still appear in the search modal
  • If you click one of those hidden items, the modal is closed, the column is marked visible, and scrolled to in the list
  • If you drag one of those hidden items, the column is marked visible and drag and drop works.
• If you drag a group item out of the modal, the whole group is dragged
• Very long column names are truncated

Probably some other things I'm forgetting, but it should be pretty robust