I made a merge request to gdk to fix the html mime<->uti problem on MacOS:
https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/7332

My idea was to fix this in gdk such that mime types are translated to the MacOS UTI style:

https://gitlab.gnome.org/fredowski/gtk/-/commit/f54f2f280827660e4360c9890beddb72c66e0b3e

This would also translate the mime "text/html" to uti "public.html". It turns out that applications have different "preferred" formats from the clipboard. You have switched off the image targets for tables with the patch mentioned below to influence the libreoffice clipboard choice. If image targets like png are available on the clipboard, then libreoffice prefers the image targets. Libreoffice also allows to choose from other formats via "Paste Special" but other applications do not have that option. So basically we decide via our advertised formats how data is imported in different applications.

It turns out that some applications prefer "html" over "images". But our "html" export for charts just mentions the image file. So we would need to disable "html" from the announced formats on the clipboard for charts. Currently advertised formats with my above gdk patch are

Charts:

• public.plain-text
• public.utf8-plain-text
• public.html
• public.jpeg
• public.png
• com.microsoft.bmp
• public.svg-image
• org.oasis-open.opendocument.text

The following applications import charts via clipboard as follows:

Libreoffice: png image
Apple Mail, Notes: A kind of empty box - I guess from the html with the link to the local file system which is not safe)
Apple Keynote: The plain or utf8 text with the name of the file of the image
Inkscape: png image

This is interesting as inkscape imports as png but without my gdk patch, the svg is advertised as "image/svg+xml" on the clipboard and inkscape imports svg then. So at least inkscape needs the svg mime advertised and ignores the svg uti. But inkscape is gtk/gdk based and would probably use the same method if fixed in gdk.

Tables:

• public.plain-text
• public.utf8-plain-text
• public.html
• public.svg-image
• org.oasis-open.opendocument.text

The applications import the following format:
Libreoffice: html table
Apple Mail, Notes: html table
Apple Keynote: svg
Inkscape: nothing

So Apple Keynote prefers the svg for tables but not for charts. Strange. Without my gdk patch, inkscape imports the svg.

Anyway, I think the mime to uti conversion for "text/html" should be done and it should be done in gdk for all applications. Maybe with a less intrusive patch such that just "text/html" is converted and not everything. Not sure.

For pspp I think the current implementation is based on wrong assumptions. We make this target list

https://git.savannah.gnu.org/cgit/pspp.git/tree/src/ui/gui/psppire-output-view.c#n717

and later add text and image targets:

https://git.savannah.gnu.org/cgit/pspp.git/tree/src/ui/gui/psppire-output-view.c#n740

However, adding a target via gtk_target_list_add_text_targets is misleading as this adds a bunch of text targets to the list:

https://gitlab.gnome.org/GNOME/gtk/-/blob/gtk-3-24/gtk/gtkselection.c#L383

what we have already done via the array. What we will receive as callback identifier like "SELECT_FMT_UTF8" is not clear. I guess the list will be searched from the beginning such that all "gtk_target_list_add_text_targets" are obsolete.

The same holds for gtk_target_list_add_image_targets. This just adds all targets which can be produced by a gdk_pixbuf. See:

https://gitlab.gnome.org/GNOME/gtk/-/blob/gtk-3-24/gtk/gtkselection.c#L454

This does not include svg, so we do in fact not add the svg target by that call. It was already in the array.

But I think the idea is correct that we want different advertised targets depending on our widget type, i.e. different targets for charts and tables.

I did some further investigation and figured out that MacOS posts the html version with the mime type "text/html" but on MacOS this requires "public.html".  With some quickfix I added "public.html" as additional advertised type here:

https://git.savannah.gnu.org/cgit/pspp.git/tree/src/ui/gui/psppire-output-view.c#n724

and then I can paste in Libreoffice with "html" which is better rendered. When I have both "img" and "html" in the clipboard, then Libreoffice prefers the image as the default pasted. So the first thing would be to have text/html advertised correctly on MacOS.

The advertised "application/vnd.oasis.opendocument.text" is not considered by LibreOffice, not on Linux and not on MacOS.

MacOS uti <-> mimetype relation is described here: https://developer.apple.com/library/archive/documentation/Miscellaneous/Reference/UTIRef/Articles/System-DeclaredUniformTypeIdentifiers.html