ChatInputResizeHandle

Render the border line beneath the drag target.

Highlight the resize target on hover.

Remove hover feedback, but never mid-drag.

A drag routinely travels outside the handle; dropping the highlight there would make the border flicker off the moment resizing starts.

Begin resizing from a left press on the handle.

Publish movement during a captured drag.

Finish an active left-button resize.

Hover feedback drops here rather than being re-derived from the pointer position: releasing the capture makes Textual recompute what the mouse is over and deliver a Leave anyway, and the next real movement back onto the handle re-highlights it through on_enter.

Abandon a drag when the app revokes this widget's mouse capture.

Without this the handle keeps a phantom drag alive: on_leave refuses to clear the highlight, and later pointer movement resizes from a stale baseline with no press behind it.

Abandon a drag when the composer is hidden mid-gesture.

Toggle expanded sizing on a double-click (or a faster chain).

Release mouse capture and hover state during teardown.

Message sent when a resize drag begins.

Message sent with the current drag delta.

Message sent when a resize drag stops, however it stopped.

Message sent when resize hover feedback changes.

Message sent when expanded sizing should be toggled.