ErichG and Mike -
Thanks for the replies.
I'm using an AOC 24B1W 1920x1080 pixel monitor. I've tried using Windows accessibility features to make the apparent size of the S-Gear display apparently larger, as well as a magnifier, but the result was too dark and blurry to read. I have tried several selected rack sizes, including those mentioned, and that made no difference in the behavior. I changed the display resolution down to 1360x768, and that increased the apparent size of the S-Gear display, obviously, but at quite a cost in display area, and additionally made the S-Gear display quite hazy. The S-Gear magnification selector still did expand the display past 125%.
Here's something:
when I select 75%, 100%, and 125% the S-Gear container grows in both horizontal and vertical directions. When I select 150%, the container grows vertically a bit, but not horizontally past its width at 125%. When I select 200%, the container grows not at all. The right limit of the display for the "American Clean" factory preset at 150% and 200% is the right edge of the numbers next to the "Amp Drive" control on the amp, while the bottom edge of the display for the same preset is about halfway down the Pro Convolver.
Today I took my computer to MicroCenter to try an ASUS PA278QV 2560x1440 pixel monitor. their ASUS monitor at that resolution was broken, so I tried an AOC monitor with 2560x1440 pixels; it behaved the same: I could view S-Gear 75%, 100%, and 125% magnifications completely, with vertical scrolling available if needed. But when I selected 150% magnification, the right side of the display went to the right edge of the application container and disappeared, with the edge as described above. No horizontal scrolling was available. Selecting 200% magnification after having selected 150% did nothing. There was plenty of screen real estate available outside the application container, though.
The Config setting for "Font size" still doesn't appear to do anything.
I was able to improve the visibility somewhat by changing the Windows display theme to "High Contrast Black" and by resetting the background image to black. I think this helped because it reduced visual clutter on the display.
Since S-Gear acted the same on two displays of different pixel dimensions, I conclude tentatively that the problem is not the display.
I have to say that I've never seen any Windows application that behaves like S-Gear - no resize arrows appear when I hover over a boundary of the container so the container can't be resized arbitrarily, and the full-screen button next to the "Quit" (X) button doesn't do anything. I suppose it's possible that something in my video drivers is at fault, though it's odd that these issues only show up for S-Gear, but not for any other Windows app I use. Honestly, I suspect there's something about the way the S-Gear application was written that makes it not play well with Windows. Perhaps the imperative that the graphics of the UI would look a certain way overruled the usual programming patterns for interacting with the OS, but I'm only speculating.
Anyway, for the moment, changing to the "High Contrast Black" theme with a plain black background has helped to some degree. But it's not particularly satisfactory.