i don't know if that part belongs in the bug report, but:

i'm getting the same thing. https://developer.blender.org/T78200

ah, this is probably a duplicate of https://developer.blender.org/T77669

More information about the bug (and workaround):

More context: the grease pencil tools do work as i expected (active tool not affected by undoing) when the operations that were carried out before undoing were all part of the same mode (e.g. drawing and erasing).

hi Antonio. What i was expecting is that the 'active' tool/mode was independent of the undo stack (and wouldn't be changed when undoing).

this is still an issue in 2.90

Sounds good. I'll file a new bug if i notice it in any other file.

I haven't noticed this problem with recent versions!

Yes. I still have the issue with the latest beta.

I also notice this same issue when dragging the playhead in the GP timeline; the playhead often gets accidentally 'dragged' after my pen-up, when i'm trying to move the cursor away from the playhead.

This sounds like a similar issue to this bug: https://developer.blender.org/T63699

Here's a .blend file that i have the problem in. I couldn't reproduce with a simpler file yet.

I agree that erasing active layer only would be a better default. This isn't a bug, but the way the eraser is designed to work currently. As a workaround you can use the layer isolate button in the layers panel to toggle locking all non-active layers and hiding them so the eraser will only affect the currently active one.

I understand. But specifying *any* default value has the same problem. At the moment 'opaque' is the default in all cases. I'm wondering whether alpha blend might be a more appropriate default (a fit with more use cases, fewer clicks for end users) in cases where the alpha button has been checked.

Ah, thanks. In 2.7, using the Blender renderer, the default effect of the above steps was to create image plane with transparent areas (as i was expecting), without needing to adjust the material further. Do you know if there's a deliberate reason that's no longer the case?

Possibly related. Blender crashes if I then do the following:

Ah seems there's still a problem. Crashed again with this complaint:

@Antonio Vazquez (antoniov) will comment to confirm after this render is done, But on frame 1000 without problems so far.

Checking console log showed that it was a problem with a 3rd party add-on (snapscrubber). Removing the add on fixed the problem.

Sorry, I can't test this. I only have access to macs at home. The double clicking problem was an issue on the windows laptops I was using in the training room at the Blender institute while attending a course there. I've never had the problem on a mac.

sorry, i'm on the GP course at the blender institute, don't know if it affects previous builds.

I understand, thanks! (a new compatible file on the cloud would be great)

Thanks. Would it make sense (is it feasible) to have some kind of error/alert appear in Blender signalling that the file the user has tried to open isn't supported in cases like this?