igloo: measure the canvas gestures on a settled panel

A panel with a fetch in flight is drawn against the range its data
covers, a storage step wider than the one it asked for. The context
view's bar was measured right after each of two drags, so the two
widths could straddle that switch; both now wait for the group to stop
fetching. The gestures also scroll their target in, addressing viewport
coordinates as they do.

The peak zoom spanned the middle half of the plot, which outward tick
alignment rounds back to the domain it started from. It now drags a
narrow band, and is held to the labels paired with their offsets:
either alone can stay equal across a narrowing.

JIRA-Ref: CMK-37844 | CMK-38016
Change-Id: I17b2ae158c040a04887ad16f4b85844e77952a06