[foxnne]

Fixes the maintainer's report on main (58cdcb1, after #653 and #656): "sliding the scrubber on the history seems to modify the snapshots of each step".

What changes

A step's picture in the history strip was taken again every time the history moved onto it: each undo, redo, historyGoTo and scrub landing re-captured the pane and replaced what the step had (fetchOrderedRemove + put in HistoryFrames.land), so sliding the scrubber changed the snapshots, their crop and change box shifting with whatever the pane showed at that frame.

Now a step's picture is taken once, as the step is made, and kept:

• The editor reads the track's records since the last move (HistoryFrames.changeOf). An added or joined record for the entry on top is the step being made: its picture is (re)taken, so a run of typing, one step whose id stays put, still ends with a picture of the whole run.
• A new track for a document that had one (a restart, Tracks.step: reload from disk, a restore) lets go of every picture: none is of the new history, whose ids may repeat the old ones.
• Anything else (undo, redo, go-to, a scrub, the "going back is a step" re-push of old entries, text: going back is a step, so an edit after it loses nothing #571) is a visit: it only fills in a picture the step lacks, compared with the step truly before it on the line (below, from historyLine), never the last position visited. A visit's picture that lands after the step got one is dropped.
• One exception keeps pictures from going stale: a visit retakes a step whose picture was taken from a different pane size (Picture.source against the pane as it stands, pane_source), the window resized since, so a strip never mixes two views.
• A retake no longer carries the old picture's change box over: the only retakes left are of a step that changed, or of another view, where the old box is wrong.

SDK impact

• None

Verified

• macOS: zig build test (new tests: scrubbing to and fro across five steps leaves every stored picture's prints, source and change as taken, and nothing is asked to be taken; changeOf tells made, joined, visited and restarted apart), plus the gates below.
• Windows:
• Linux: CI test-integration.
• Web: zig build check-web.

Follow-ups

None.

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