Wrong insert position when editing ligatures in rich text boxes

[anydream]

Wrong represent by GacUI:

Correct represent by Windows notepad:

The font is Calibri from Windows 10 system.

[vczh]

Will fix after the document work is done
Reference

[anydream]

Some of OpenType fonts have ligature caret position in GDEF table:
https://docs.microsoft.com/en-us/typography/opentype/processing-part2#gdef-table

[anydream]

And the correct way to split characters from ligatures:
http://unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries