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I am using the previous release of Geany 1.23.1, and not the last one, I don't know if changed something about this?
The autocomplete box is generated by Scintilla (another project whose editor widget we use). There were quite a lot of changes between 1.23.1 and 1.24.1 the current (and already a year or so old) release. Too long ago to check if they explicitly addressed this, but the box does re-size to fit the longest item for me in 1.24.1.
Cheers Lex
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