Richard, a browser window will dynamically downsize the images to fit the screen if you make the horizontal dimension of the browser smaller. See images below:
Here is a browser in full screen mode on a 1920x1080 laptop.. as you indicate, the image is larger than the screen can accommodate and you have to scroll to see the bottom of the image.
Now, here is the same image when I exit full screen mode on the browser and shrink only the x dimension of the browser window. After a shrink it a bit below about 75% of the screen width or so, the image dynamically shrinks with width of the browser and you can then size it to whatever size you want to fit the screen without having to scroll. See below:
If you use a browser that allows you to move your tabs to the left of the window instead of at top, it will give you even more room in the y direction, so you wouldn't have to shrink the window as much. Now of course, this works well for images that are of a fixed size, in my case above, 1600 px on the long edge. If all of a sudden you get an image that is much smaller, or where the y dimension is 1600 px (the forum allows you to use images up to 1600 x 1600 if you wish), then of course you will need to resize the window again. There is no provision to dynamicall resize the display to maximize the screen use depending on the size of the image. That's what the letterbox is supposed to do but of course, for that, you need another click as you observed.
We will look into this further to see if there are better solutions. Maybe
@DavidWatts can also comment.
