Colours can't be darkened if the default font is large

[SeaLiteral]

Steps to reproduce:

1. Set a web browser's default font size to something large. Exactly how much depends on OS settings and maybe screen resolution. If the OS is set to use a large font by default (as is the case on the laptop where I found this problem), the issue may happen even with a browser's default font size. Alternatively, I suspect zooming in might cause this issue as well.
2. View some captions.
3. Try to change the background colour.

Expected results:

The colour picker works, and any colour can be chosen.

Actual results:

Only a part of the colour picker is shown, with a scrollbar to show the bottom of it. Trying to click or drag the scrollbar with a mouse will just close the colour picker, and using the scrollbar only reveals the middle, the bottom can't be scrolled to and instead you see other parts of the page starting to scroll. The top of the colour picker does work, but without the bottom, there are limits to how dark colours can be selected. I first noticed this issue on the background colour, but it seems to happen on the text colour too.

Ugly workaround:

Zooming out (control-minus) reduces the need for the scrollbar and can get the bottom to be shown while the scrollbar is still there. Or one could keep zooming out further and not need a scrollbar at all. But having to zoom out isn't ideal, even if you can zoom in again after changing the colour.

Platform

I've got this problem using Google Chrome on Windows 10. I'm not sure if other browsers or platforms are affected.