meson -Dprefix=$PWD/_install _build
cd _build
ninja
ninja install
I then made a simple change to the help output such as added gibberish to the description and the --diff help text.
I then execute ninja in _build and I get: ninja: no work to do. I execute ninja install and running _install/bin/meld --help does not yield the changes made to the help text.
I execute meson compile in _build and I get the following:
Found runner: ['/usr/bin/ninja']
ninja: Entering directory `.'
ninja: no work to do.
I am not exactly sure how development in meld is expected to work, but you could try running the bin/meld binary from the repo directly and it should work, the installed one will try to import the meld module which my guess is ends up opening your system install.
Alternatively you can use GNOME Builder, with flatpak, and everything should work out of the box.