Column shell: a drag sets off from the wider of the width drawn and the owner's, so a column a short shell squeezes moves with the pointer, where it jumped back against it, and a drag with no sideways move changes no width, where it wrote the squeezed one

AGENTS.md

@@ -221,7 +221,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - `typography::Ellipsis` shapes its line with the text style's own run, so a strike or an underline set on its box shows (`lists::ListItem::struck`, a closed task's title).
 - Focus that must follow an item to another parent is taken by the owner and keyed by the item: a handle gpui makes for an element dies with its path (`project::KanbanBoard`, whose cards keep focus across columns).
 - The theme's `platform`, the system's to start, decides the window buttons Ely draws, the title bar's layout and key caps, `secondary` included (`typography::keys::keystroke`). A title bar whose buttons the OS draws follows the OS.
-- A window whose sidebar runs to its top is `shell::ColumnShell`: each column's header drags the window; the first leaves the traffic lights room, and drawn controls lead the first header in the macOS manner or end the last. A set width is a flex basis, not a width, so it holds its header's own parts, which a width let run past it, and yields when the shell runs short; the host's header items keep one control's width and scroll when short. A resizable column drags by its border toward the filling column, its right before it and its left after it; the handle draws that border and occludes the header under it, and a drag sets off from the width drawn, not the owner's.
+- A window whose sidebar runs to its top is `shell::ColumnShell`: each column's header drags the window; the first leaves the traffic lights room, and drawn controls lead the first header in the macOS manner or end the last. A set width is a flex basis, not a width, so it holds its header's own parts, which a width let run past it, and yields when the shell runs short; the host's header items keep one control's width and scroll when short. A resizable column drags by its border toward the filling column, its right before it and its left after it; the handle draws that border and occludes the header under it, and a drag sets off from the wider of the width drawn and the owner's, since a header's parts may hold a column wider and a short shell squeeze it narrower; a drag that has not moved sideways changes nothing.
 - Taffy counts a flex item's text in its least width whatever width the item sets. A box that must add none lays its content absolutely inside a filling box (`shell::ColumnShell`'s title and body).
 - A centered title sits between a start and an end that grow alike from a zero basis, their padding on a box inside them, since taffy counts padding in the flex base; their content is their least width, so the title gives way before either end (`shell::TitleBar`).
 - A pane the window's blur shows through is `shell::Vibrancy`, filled with the palette's `glass`, in a window opened `WindowBackgroundAppearance::Blurred`: vibrancy on macOS, acrylic on Windows. gpui's `MicaBackdrop` and `MicaAltBackdrop` are DWM backdrops from Windows 11 build 22621; older builds get none, in silence, and macOS plain transparency.