In T67442#730340, @Brecht Van Lommel (brecht) wrote:

This is just not supported, for modifiers and constraints to work well objects should not have zero scale.

I mean object disappears as soon as one of its dimensions reaches the value of 0, even if mouse button hasn't been released. Blender 2.79 accepts input only after user releases the LMB.

I am unable to reproduce, see the attached clip. Pressing the right mouse button cancels scaling for me.

Windows 7
GeForce GTX 470
RC2

You can see that object disappeared even though after I reached the value of 0 I didn't release the button and continued scaling it in opposite direction.

This seems to be a problem with undo:

• Open the example blend file.
• See that the armature object is selected.
• Follow the steps to reproduce (so cube has zero dimension, even though right-click was used to cancel).
• The actions so far were: select the cube, change its dimensions, cancel changing its dimensions.
• Press Ctrl+Z.
• The armature object becomes selected, but the cube is still invisible.

To me this means that the right-click was indeed interpreted by Blender as a form of undo. However, it was incomplete as the cube still has zero scale. Manually setting the scale back to 1 makes the cube visible, so this is not something that's impossible for Blender to do. However, restoring the zero-scale situation requires either a real undo or a change in a different property than was dragged. This might be a limitation of the current UI system?

• Undo has nothing to do here, right-click is supposed to reset state of object as it was before init of transform operation, without any undo or redo. it just cancels current WIP changes.
• It is indeed a known limitation, dimension is not an actual property, but a virtual value computed from scale, parenting, etc.

It is probably fixable one way or another, but almost certainly not trivial. so will consider as known issue for now.

This is caused by the armature modifier results with non-invertible matrices.

The mesh gets transformed by the modifier into a single point, the new bounding box is then used to calculate dimensions - which can't give a useful result.

I looked into using invert_m4m4_safe/safe_ortho, while it does resolve this bug, the results from the modifier are quite unreliable/glitchy.

Another solution could be to store the objects bounding box at the point where editing dimensions starts, then re-use it for further dimension calculations.
Currently the UI API doesn't support custom begin/end events, although they could be added, it would only resolve the issue during a single interactive drag action.

Moving this to a TODO.