This is for geany 1.38 on MacOS.
On the Mac, the commonly used key combination for placing the cursor to the start and end of the line is
command+cursor_left : Start of line command+cursor_right : End of line
Command+cursor_right does not seem to have any effect in Geany. Checking the Keyboard assignments in the Geany Preferences, I can see that "Cursor start of the line" is assigned to "Home" and "Cursor end of the line" is assigned to "End".
The keys "Home" and "End" don't have a representation on the Mac keyboard. Of course I can re-assign those two functions to what I like, but I suggest that Geany for the Mac initially uses those key assignments which are commonly used on the Mac.
Unfortunately we currently don't have a way to specify different keybindings for Linux, Windows, or macOS so they are identical for these platforms (not a big problem for Linux/Windows where the defaults don't differ too much but a bigger problem for macOS where the default keybindings are really different). There's an unfinished work in progress PR here:
https://github.com/geany/geany/pull/1395
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