Seconding Millie’s recommendation of “landscape” and “fullscreen” modes.
Decker’s default deck size is very much landscape-oriented, so it’s never going to work very well on a portrait-mode device.
In addition, Decker tries to display itself at an exact multiple of the deck size, to keep those pixels razor-sharp for the retro aesthetic. However, in full-screen mode Decker abandons that restriction and displays itself as large as will fit. For example, an iPhone 16’s screen is 1179px on the short axis, which is ~3.4 times larger than a Decker deck. Normally Decker will scale itself 3x, leaving a wide black margin (about 13% of the screen height, total) around the edges. In fullscreen mode (when you pick “Fullscreen” from the Decker menu), it expands to cover the full height, giving you a bit more space to poke at buttons and drag sliders around.