GTK: Unexpected whitespace rendering in some fonts

[Robin Haberkorn]

Dear Scintilla hackers,

I noticed the following peculiarity in GTK 3 builds (reproducible in the HEAD
commit): When using the "Terminess Nerd Font" [1] with and without 1.5-width
icon glyphs, leading spaces get squashed unexpectedly when inserting the
overlarge Unicode glyph 0x2739 (✹ ). Attached is a screenshot demonstrating
the issue in Bait.

Any idea why this might be?

Best regards,
Robin

[1]: https://github.com/ryanoasis/nerd-fonts/releases/download/v3.4.0/Terminus.zip