Here's the list of everything that needs to be done to make animated transform masks work.

In the time of writing (August 30th) I'm on this commit: d1442548b17a1d251e0d995f82f2a457d88046fd

• Uncomment the code to add transform masks frames

• Bug https://bugs.kde.org/show_bug.cgi?id=402841 makes it not possible to playback animated transform masks, It shows correctly when you click on the frame though.

The workaround for the animation cache bug was to create a layer with a lot of duplicate frames, add the transform mask, add frames and then merge together. But:

• Transform Masks right now (??? there were a lot of changes to the transform tool in winter 2019) crashes when you want to merge them with a layer below. To make it not crash, revert the commit: 9a7cb4bdac690d9dec7cdb820a95d1bc922abdfc

• Animation curves are hard to work with. - https://bugs.kde.org/show_bug.cgi?id=399120

• Transform animated keyframes is only working with some specific modes, mainly the Free Transform (didn't check with the Move Tool, but Move Tool is a bit useless in this case anyway).

• Transform keyframes can be saved to files (and those files open in current version of Krita) but they don't work - even the workarounds are not working correctly. There is just no updating of the content of the layer. (I couldn't figure it out in time I had for this task.)

• Merging an animated layer with transform mask can give you wrong results in some cases: https://bugs.kde.org/show_bug.cgi?id=377491 (this needs to be checked again, too)