oh, block caret.
Selection and display of the caret is performed by the [Scintilla](https://sourceforge.net/p/scintilla/bugs/) editing component Geany uses, please check it with Scite, the Scintilla reference editor and report upstream if not already.
Note that IIUC block caret is not supported by the Scintilla maintainer, but an external implementer, and it hasn't had much love recently.