Let me start out by saying that I am not sure if this is a common issue, or a rare issue. My current workstation has been upgraded from Windows 7 > 8 > 8.1 > 10 and I have had a some strange issues crop up, so there is a good chance this problem is somewhat unique. To summarize the problem:
Certain Unicode font glyphs were not showing up in certain programs, but displaying fine in others. An example would be that Chrome/Firefox worked fine, but MobaXTerm and Notepad++ did not. The issue was related to Windows Font Fallback and/or Font Linking.
The way the issue manifests itself is when you attempt to display Unicode characters that do not exist in the now selected font, they display as missing characters, either a ? or a box. The correct/desired functionality is for windows to fallback to another font which can display the missing glyphs based on a list of which fonts have the missing glyphs.
For example, when I was customizing my WeeChat configuration, using Notepad++ the glyphs I wanted to use were displaying as missing, a snippet of that configuration is below:
Many of the special glyphs in this snippet were not showing up in any of the usual mono space fonts on my system including: Consolas, Source Code Pro, Inconsolata, Deja Vu, but only in a few programs. If I viewed them in Firefox or Chrome, they showed up fine. Upon further investigation I figured out that the missing glyphs are not in the base mono space font, but are provided by the Segoe fonts on windows, specifically Segoe UI, Segou UI Symbol, Segoe UI Emoji, Segoe UI Historic, Segoe MDL2 Assets. Some programs would do this fallback, others would not.
A specific example of this issue, in Notepad++ can be found in this github issue.
It appears that perhaps some programs have built-in special ways to handle this situation, and others have not. In the programs which do not support this, you would think the OS would deal with it a bit better but unless my registry is messed up as noted in the first paragraph of this post, it seems that some manual workaround is needed.
The way I wound up resolving the issue was found in this article on Windows Globalization. Basically I defined registry entries for my mono space fonts and defined the linked fallbacks manually. The path to these keys in the registry is:
Within that part of the registry you create a new key of type Multi-String Value, the name of the key should be the name of the font as is found in the list of system fonts:
Remove the font type from the name, for example “Consolas (True Type)” would become “Consolas”
Set the value of the key created to the list of fonts you want to use as fallback, in the order you want them to fallback, for example:
I have attached a .reg file you can use to create entries for Consolas, Source Code Pro, Deja Vu, and Inconsolata with Segoe fallbacks, use at your own risk!