Bugzilla

### Beta/Release Uplift Approval Request
* **User impact if declined**: Recently-assigned Unicode characters (e.g. "CJK Extension I" block) may not be usable even if the user has installed a font that supports them
* **Is this code covered by automated tests?**: No
* **Has the fix been verified in Nightly?**: No
* **Needs manual test from QE?**: Yes
* **If yes, steps to reproduce**: On Windows 10:
(1) install [jigmo](https://kamichikoichi.github.io/jigmo/) fonts (see comment 0)
(2) visit https://en.wikipedia.org/wiki/CJK_Unified_Ideographs_Extension_I
- Note that the table of characters shows missing-char blocks
(3) toggle pref `gfx.font_rendering.fallback.unassigned_chars` to true
- Confirm that the table of chars now shows the proper glyphs

(similar behavior is likely to be seen on other platforms as well)
* **List of other uplifts needed**: None
* **Risk to taking this patch**: Low
* **Why is the change risky/not risky? (and alternatives if risky)**: Trivial change to add a (default-disabled) pref allowing font fallback to kick in here
* **String changes made/needed**: 
* **Is Android affected?**: Yes

### Beta/Release Uplift Approval Request
* **User impact if declined**: Recently-assigned Unicode characters (e.g. "CJK Extension I" block) may not be usable even if the user has installed a font that supports them
* **Is this code covered by automated tests?**: No
* **Has the fix been verified in Nightly?**: No
* **Needs manual test from QE?**: Yes
* **If yes, steps to reproduce**: On Windows 10:

(1) install [jigmo](https://kamichikoichi.github.io/jigmo/) fonts (see comment 0)

(2) visit https://en.wikipedia.org/wiki/CJK_Unified_Ideographs_Extension_I
   - Note that the table of characters shows missing-char blocks

(3) toggle pref `gfx.font_rendering.fallback.unassigned_chars` to true
   - Confirm that the table of chars now shows the proper glyphs

(similar behavior is likely to be seen on other platforms as well)
* **List of other uplifts needed**: None
* **Risk to taking this patch**: Low
* **Why is the change risky/not risky? (and alternatives if risky)**: Trivial change to add a (default-disabled) pref allowing font fallback to kick in here
* **String changes made/needed**: 
* **Is Android affected?**: Yes