Selections in the viewport are always of a single type. Eg, all bones, mesh attachments, region attachments, etc. The only exception is the Create
tool, which selects bones but allows ctrl
(cmd
on macOS) clicking attachments.
It's similar in the tree: while it allows selections to have items of different types, most operations require all selected items to be of the same type. Drag and drop requires items to be the same type and you'll see more properties in the tree when all selected items are the same type. When items are different types, usually the only operations available are delete, duplicate, and rename.
Why are you needing to make a selection of every item? If your goal is to reverse an animation, press escape
or spacebar
so nothing is selected which makes the dopesheet show all rows. Box select the first row, then drag the left side of the box selection to the right, past the right side of the box selection. This will flip over the selection, reversing the keys. Keep dragging until the selection is the same size as the original and you'll have the same timing.
There are a few gotchas. Make sure the dopesheet is not locked and is not filtered. Hiding slots and bones also hides their rows in the dopesheet, so you can use ctrl+H
(show all bones/slots) to make sure none are hidden. Lastly, attachments in skins can have deform keys and when the skin is not active, the dopesheet rows don't appear. To solve this, click the tree view settings button and check "Show all skin attachments":
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That makes the tree show all the attachments for every skin, but it also makes the dopesheet show the rows for all attachments that are in skins, even for those skins that are not the active skin.