Add support for scroll position sync

[guanglinn]

Add basic support for scroll position sync between Markdown preview (view-mode) and its source code (edit-mode).

This feature supports:

• Sync position from edit to preview;
• Sync position from preview to edit.

The related issues:
#2373 #2189 #1070 #1060 #679.

[guanglinn]

You can see the demonstration ↓

2024-09-03-markdown-scroll-sync.mp4

[gsantner]

has conflicts

[guanglinn]

Conflicts are resolved :)

[gsantner]

Tried locally,

unfortunately not works quite well for both directions. From View-Mode to Edit-Mode it seems to works good enough.
But from Edit to View mode it seems to not work, always jumps to top of document.
This is the builtin Markdown reference.

screenrecord-2024-09-25_00.59.29.webm

[gsantner]

I've merged your other PR to master as well, made conflicts again

[guanglinn]

Conflicts are resolved now.

[guanglinn]

Sure, from Edit to View, it works well for quite a lot common text blocks such as Headings, Bold, Italic, Blockquotes, Lists, Links, Images, ..., but it works not quite well for some special text blocks.

I have tried to solve this, it's quite difficult for myself, and I think this is already a big improvement for Markor to provide a rough position synch for Markdown.

As mentioned above, you can merge it first, it is stable now, and I will improve it if I have some better solutions later, or other contributors would make it perfect on this basis.

[gsantner]

What I would suggest for now: Save/Restore last webview scroll position after short delay (when blocks have the right rendered size).

As far I recall this has been before like that, but somehow it doesn't work currently on master either

[guanglinn]

Can an option be added? Allow users to choose to let Markor restore last WebView scroll position, or sync WebView scroll position from Edit.

[gsantner]

I don't think it's neccassary. When scroll sync work good enough in praxis there is also the editor position saved/restored - so will be view-mode position when switiching to.

[gsantner]

Unfortunatley also keep in mind what I tried to say always - Markor is not only Markdown. This all relies on Markdown specific injections/adjusting the original.

This won't work for stuff that doesn't base on Markdown converter. So we need in any case such a fallback. And saving/restoring the view-mode scroll position should be viable for that.

It can't be resolved generically for everything and not easy to get much stable.

[guanglinn]

Improvements: If we want to go back to the last view position, just long click the toolbar.