Re: Percentage Bar
Yes, the lack of color scheme for the layer is the problem. A 'defaultScheme' should have been created automatically the first time you started the graphics composer, not sure why it has been removed. Without a color scheme, the library does not have a color to draw the background layer with, thus the previous rendered image persists. This is evident when you reduce the width, but not when you increase the size (since new image section is drawn).
To add a scheme, click the "+" sign in the schemes window on the lower left corner of the composer window. You can change the colors to your preference. Assign that scheme to the layer. For some reason, I'm unable to assign a scheme to myLayer0 in your project. So I just added a new layer, assigned the new scheme to the layer, moved the widgets to the new layer and removed the old one. I also switched the background type of the image widget to "None".
With these changes, I can see the image width reduced when I press the button. Give the above steps a try and see if it works for you.