Whenever I am imaging I use RealVNC on an iPad to connect to my KStars/Ekos instance running on a remote Linux machine. The KStars FITS Viewer has those nice zoom buttons and unfortunately they are missing in the DSLR Image Viewer. I am to understand that zooming can be performed with the middle mouse button but ... there is no mouse on an iPad. Using one or two fingers and sliding on the screen doesn't work. The possibility to zoom in would come VERY handy when focusing with a Bahtinov-mask. Would it be possible to add those buttons to the DSLR Image Viewer please?
Not sure if this will help. I use the focus tab with my bahtinov mask and then zoom in while using the framing function. I know it is not what you are asking for but it may help.