A content-less layer, separate from the SheetCellEditorLayer that paints the box. The box
sits at the top of the stack to paint over everything, but a Layer's handlers are its slot's
tap — grid-wide ears that fire on every layer's turn, top first — so start handlers on the top
layer intercepted keys and double-clicks aimed at a chart or note before that widget's own handlers
ran. Starting an edit is the grid floor's behavior: WorksheetElement mounts this as the first grid
child, where it only hears what no widget above consumed.
Keys are positionless, so the floor slot alone is not enough: a focused widget that does not consume a printable key (it must let keys through so its own commands can resolve) would still reach this tap. The key start therefore acts only when the key is aimed at the grid surface itself — the surface, or the pane's focus target standing in for it (the VirtualKeyboardLayer's input), is the focused element. A double-click has a position; a widget that owns it consumes it.
Text that reaches the focus target without a keydown that started an edit (a virtual keyboard can deliver it that way) starts one too, seeded with the text.
The sheet's ready-mode edit starters — the
@sheetxl/reactcounterpart of grid-react'sEditorStartLayer: a typed character /F2/Backspaceopens an edit on the anchor, a double-click opens one on the cell under the pointer, and a rejected start raises the Excel-style heads-up (protected sheet, array part, unlock-range prompt).