The appearance changes of the instances on the layers now follow the edited objects through the same pass as the instances in other symbols, graph designs, pattern tiles and brushes. That pass also reaches instances used as opacity-mask art, which keep their changes on the same objects when an edit reorders them and drop the change to an object the edit deletes.
An override on a dynamic symbol instance names an object of its symbol's
art by path. Replace Symbol kept every override whose path the new
symbol's art also had, so a change made to one object of the old symbol
was applied to an unrelated object of the new one. An instance given a
different symbol now drops its overrides; an instance replaced with its
own symbol keeps them. Undo brings them back.
symbol.delete {names} expands every instance of the named symbols,
round by round, before it removes any definition, so a symbol and the
one placed inside it delete alike in either order. The Symbols panel's
picks belong to the document they were made in, and a single picked
symbol is deleted only while symbol.unused still lists it.
symbol.unused lists the symbols nothing places: instances in the art
(opacity masks included), graph designs, pattern tiles and brush art
count, and so do instances in a used symbol's art. The Symbols panel
menu's Select All Unused shows them selected, and Delete Symbol asks
once and deletes those still unused in one undo step
(symbol.delete {names}).
A menu under each of the five tiles in Pattern Brush Options lists None,
Original and the document's pattern swatches; the corner menus keep the
four automatic corners. A swatch choice copies the swatch's whole art and
its tile frame into the brush, so the brush keeps drawing after the swatch
is deleted, and the menu then shows that art as Original. Side cannot be
None: OK shows "You must choose a side pattern." and keeps the dialog open.
brush.options takes side_tile, outer_corner_tile, inner_corner_tile,
start_tile and end_tile (a pattern name, none, original, the corner auto
ids, or {"pattern": name} for a swatch named like one of those words), and
rejects side_tile "none" and unknown names before editing.
The question for brushes no stroke uses reads "Delete the brush selection?" with Yes and No, from the bottom button and the panel menu, and Alt-click (Option-click on macOS) on the button still deletes them without asking. When a stroke uses one of the brushes, the button, Alt-click and the menu item ask with Remove Strokes and Cancel; Remove Strokes deletes the brushes and the strokes that used them become plain strokes.
brush.unused also counts the tile that pattern editing's Cancel puts back, which brush.delete already clears, so the panel asks the in-use question for a brush only that tile uses.
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