The floating toolbar and the emoji panel hardcoded "Segoe Fluent Icons",
which ships with Windows 11 only. DirectWrite substitutes a font silently
instead of failing, so on Windows 10 every private-use codepoint the toolbar
draws (U+E982, U+F110, U+F138, ...) rendered as a blank box: the buttons
still worked but no longer showed any state.
Resolve the font per codepoint rather than per family, because "Segoe MDL2
Assets" is a subset of Segoe Fluent Icons and swapping the family alone would
still draw tofu wherever a glyph is missing:
- add msimeui::ResolveIconGlyph(), which probes the installed icon fonts with
IDWriteFont::HasCharacter() and returns the first family that really carries
the glyph, or no family at all;
- give every toolbar and tab icon a text label to fall back on (中/英, 半/全,
简/繁, ...) when neither font has the glyph;
- drop the unused ToolbarIconButton::SetGlyph/SetLabel, which would bypass the
resolved family.
Add unit tests for the resolver, covering the missing-glyph path that cannot
be reached on a host that has Segoe Fluent Icons installed.
fanlusky