If I write a long line to STDERR during a compilation the line is broken in the compiler tab at column 8192. This makes it impossible to pick up a copy of that line as there is no facility for copying multiple lines from the compiler tab - only one or all? Please could you make it possible to copy two lines from the compiler tab - or - an option to prevent lines being broken? Thanks!

AFAICT there is nowhere that Geany sets a limit of 8192, so it is probably internal to GTK and out of our control.
The single line select is because of the go to line functionality.
As has been noted before, Geany is targetted at normal human written code, outliers such as minimised HTML on one line or very long outputs from compiler commands may not be well supported.
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