nix: make Qt6 work on wayland

Taking many tips from our `default.nix` (thanks @nt!) this seems to be
the minimal overlay required for a flake to get up and running with
`piker chart` B)

Notes,
- for now, we're pinning to a major `cpython` version (3.13)
- ensure we (can) build with `nixpkgs.qt6.qtwayland`
- add the minimal Qt ld-lib-path linkings including those for plugin
  use (required for wayland mode).
- for now, hardcode "wayland" platform-mode and the linux standard
  "xdg-shell" integration.
- leave some TODOs to better parameterize around py versions.

flake.nix

@@ -19,26 +19,71 @@
         system:
         let
           pkgs = nixpkgs.legacyPackages.${system};
+
+          # do store-path extractions
+          qt6baseStorePath = lib.getLib pkgs.qt6.qtbase;
+          # ?TODO? can remove below since manual linking not needed?
+          # qt6QtWaylandStorePath = lib.getLib pkgs.qt6.qtwayland;
+
+          # XXX NOTE XXX, for now we overlay specific pkgs via
+          # a major-version-pinned-`cpython`
+          cpython = "python313";
+          pypkgs = pkgs."${cpython}Packages";
         in
         {
           default = pkgs.mkShell {
-            packages = [
-              pkgs.python313
-              pkgs.uv
+
+            packages = with pkgs; [
+              qt6.qtwayland
+              qt6.qtbase
+
+              uv
+              python313  # ?TODO^ how to set from `cpython` above?
+              pypkgs.pyqt6
+              pypkgs.pyqt6-sip
+              pypkgs.qtpy
+              pypkgs.qdarkstyle
+              pypkgs.rapidfuzz
             ];
 
             shellHook = ''
-              unset PYTHONPATH
+              # unmask to debug **this** dev-shell-hook
+              # set -e
 
-              # expose C++ stdlib to overlay
-              export LD_LIBRARY_PATH="${pkgs.stdenv.cc.cc.lib}/lib:$LD_LIBRARY_PATH"
+              # set qt-base/plugin path(s)
+              QTBASE_PATH="${qt6baseStorePath}/lib"
+              QT_PLUGIN_PATH="${qt6baseStorePath}/lib/qt-6/plugins"
+              QT_QPA_PLATFORM_PLUGIN_PATH="$QT_PLUGIN_PATH/platforms"
 
-              # always use the ./py313/ venv-subdir
+              # link in Qt cc lib paths from <nixpkgs>
+              LD_LIBRARY_PATH="$LD_LIBRARY_PATH:$QTBASE_PATH"
+              LD_LIBRARY_PATH="$LD_LIBRARY_PATH:$QT_PLUGIN_PATH"
+              LD_LIBRARY_PATH="$LD_LIBRARY_PATH:$QT_QPA_PLATFORM_PLUGIN_PATH"
+
+              # link-in c++ stdlib for various AOT-ext-pkgs (numpy, etc.)
+              LD_LIBRARY_PATH="${pkgs.stdenv.cc.cc.lib}/lib:$LD_LIBRARY_PATH"
+
+              export LD_LIBRARY_PATH
+
+              # RUNTIME-SETTINGS
+              #
+              # ------ Qt ------
+              # XXX, unmask to debug qt .so linking/loading deats
+              # export QT_DEBUG_PLUGINS=1
+              #
+              # ALSO, for *modern linux* DEs,
+              # - maybe set wayland-mode (TODO, parametrtize this!)
+              #  * a chosen wayland-mode shell-integration
+              export QT_QPA_PLATFORM="wayland"
+              export QT_WAYLAND_SHELL_INTEGRATION="xdg-shell"
+
+              # ------ uv ------
+              # - always use the ./py313/ venv-subdir
               export UV_PROJECT_ENVIRONMENT="py313"
-
               # sync project-env with all extras
               uv sync --dev --all-extras
 
+              # ------ TIPS ------
               # NOTE, to launch the py-venv installed `xonsh` (like @goodboy)
               # run the `nix develop` cmd with,
               # >> nix develop -c uv run xonsh

piker/config.py

@@ -261,7 +261,7 @@ def load(
         MutableMapping,
     ] = tomllib.loads,
 
-    touch_if_dne: bool = False,
+    touch_if_dne: bool = True,
 
     **tomlkws,
 
@@ -270,7 +270,7 @@ def load(
     Load config file by name.
 
     If desired config is not in the top level piker-user config path then
-    pass the ``path: Path`` explicitly.
+    pass the `path: Path` explicitly.
 
     '''
     # create the $HOME/.config/piker dir if dne
@@ -285,7 +285,8 @@ def load(
 
     if (
         not path.is_file()
-        and touch_if_dne
+        and
+        touch_if_dne
     ):
         # only do a template if no path provided,
         # just touch an empty file with same name.