Allow creating new folders from directory choosers on Linux and macOS

[amitbar05]

Problem

On Linux none of the "choose a folder" dialogs offer a way to create a new folder — e.g. File → Open, "Add DCP"/"Add folder", the New Film "Create in folder" browser, and the KDM/DKDM output folder pickers. The same applies on macOS. On Windows the native folder picker always shows a "New folder" button, so the platforms are inconsistent.

Cause

The dialogs are created with wxDD_DIR_MUST_EXIST, and the wxDirPickerCtrls use wx's default style, which includes wxDIRP_DIR_MUST_EXIST. wxWidgets maps this flag to disabling folder creation in the native chooser:

• wxGTK (src/gtk/dirdlg.cpp): gtk_file_chooser_set_create_folders(GTK_FILE_CHOOSER(m_widget), !HasFlag(wxDD_DIR_MUST_EXIST)); — this hides GTK's "Create Folder" button and the context-menu "New Folder" entry.
• wxOSX (src/osx/cocoa/dirdlg.mm): [oPanel setCanCreateDirectories:YES] only when the flag is absent.
• wxMSW: the flag has no effect on the folder picker — "New folder" is always shown.

A folder chooser can only ever return an existing directory, so the flag's only practical effect is hiding the New Folder affordance on GTK and macOS.

Change (first commit)

• Drop wxDD_DIR_MUST_EXIST from all wxDirDialog/DirDialog call sites (where it was the entire style, use wxDD_DEFAULT_STYLE).
• Pass wxDIRP_DEFAULT_STYLE & ~wxDIRP_DIR_MUST_EXIST to the wxDirPickerCtrls (this keeps wxDIRP_USE_TEXTCTRL on the platforms whose default includes it).
• full_config_dialog.cc and grok/gpu_config_panel.h were passing wxDD_DIR_MUST_EXIST as the second constructor argument, i.e. as the window ID rather than a style; these now use wxID_ANY and an explicit style.

Testing (first commit)

Ubuntu 26.04, wxGTK 3.2.9: verified before/after on the player's "Select DCP to open" dialog — the "Create Folder" button appears with the change. Also verified with a minimal wx test program that this flag alone toggles the button on this stack, and that the picker-based dialogs ("Select a directory") are governed by the same property. Windows behaviour is unchanged (the flag was already ignored there).

Update — second commit

The claim above that picker-based dialogs are governed by the same property as a plain wxDirDialog turned out to be wrong for GTK, caught by testing the real built app rather than only a synthetic harness. On wxGTK, wxDirPickerCtrl is backed by a native wxDirButton (wx/gtk/filepicker.h) wrapping GtkFileChooserButton, which unconditionally forces wxDD_DIR_MUST_EXIST on its browse dialog regardless of the picker's own style:

// GtkFileChooserButton does not support GTK_FILE_CHOOSER_CREATE_FOLDER
// thus we must ensure that the wxDD_DIR_MUST_EXIST style was given
long GetDialogStyle() const wxOVERRIDE
{
    return (wxGenericDirButton::GetDialogStyle() | wxDD_DIR_MUST_EXIST);
}

So every wxDirPickerCtrl-based folder field (New Film's "Create in folder", the Preferences folder pickers, KDM/DKDM output folders, the GPU binary location) was still stuck without "New Folder", regardless of the first commit's style change — this is a structural limitation of GtkFileChooserButton, not something fixable via style flags.

DCP-o-matic already has its own DirPickerCtrl (a plain button that pops a real wxDirDialog, which the first commit's fix does work correctly on) for exactly this reason on Windows (DCPOMATIC_USE_OWN_PICKER). This commit extends that macro to __WXGTK__ too, and adds the own-picker branch to the two files that didn't have one yet (locations_preferences_page.cc, grok/gpu_config_panel.h). macOS is untouched: wxDirPickerCtrl there falls through to the generic wxGenericDirButton implementation, which has no such override, so the original style-flag fix already works fine there.

Verified end-to-end on the real built app (not just a synthetic harness), Ubuntu 26.04 / wxGTK 3.2.9: New Film's folder picker, Preferences → Locations, and Preferences → GPU all show "New Folder" and support Ctrl+Shift+N now.

🤖 Generated with Claude Code

[cth103]

Is this just about fixing an inconsistency between platforms? Why would you want to create a folder when you have decided to add a DCP to a project?