Because of my poor eyesight (which cannot be improved by spectacles), I am testing on a huge display with a resolution of 3840 x 2160, and I have set System Settings - Displays - Scale to 300%.
According to xwininfo, this results in a Signal window that is 3040 x 2090 in size. Unfortunately, this causes the bottom 6% of Signal's window to be off the bottom of the screen, which prevents it from showing what I am typing in the text field when sending a message because Gnome occupies the top 200 or so pixels with its Activities menu bar and the window title bar (which is not included in the 2090).Attempts to resize the Signal window fail: Gnome refuses to let me size it less than 2090 pixels high. I am also not able to maximise the window to fill the screen (the Maximise command is greyed out when I press Alt-Space, and nothing happens when I double-click the title bar).
In main.js there is a setting MIN_HEIGHT = 550 but I'm not sure how that 550 ended up being 2090 (it seems more is at play than simply multiplying it by 300%). I have not yet figured out how to build a modified version so I can see if reducing that MIN_HEIGHT would solve the problem.
If I were designing Gnome I would have made the Scale multiplier apply to everything except minimum window sizes, to work around apps that enforce them without realising what happens when Scale is applied (there are quite a few apps out there that do this, for example Zoom). But until the Gnome team are able to change it on their side, I think it would be polite for all app writers to consider setting their minimum size smaller: perhaps try reducing that 550 to 400 and check the Signal window still works when resized down to its new minimum height? as I hope this would make it more usable on a Gnome desktop when Scale has been applied to the desktop.
In the case of this particular monitor, I am able to partly work around the problem by holding down the Super key (also known as the Windows key) while clicking on the window and dragging it: this lets me move the window slightly off the top of the screen (so that its titlebar is not visible and its menubar is only half visible), so that the "Send a message" box becomes visible at the bottom. Alternatively I can use the Alt-F7 shortcut to start moving the window with the arrow keys, and push it partly off the top of the screen in the same way. But if the monitor were any smaller, then I'm not sure that workaround would be enough..