Partially Adresses #2491. Refactors how Focus works

[tig]

This PR paves the way for fixing #2491 by addressing deep design and implementation problems with how Focus currently works.

This PR used to be way more ambitions... it was going to eventually fully enable any View to work Overlapped, removing the need for Toplevel and IsOverlappedContainer. That will now come later.

Fixes

• New design and implementation for how Focus works.
• Partially Addresses Get rid of Toplevel - Introduce Runnable and Overlapped instead #2491
• Fixes Proposal: Focus/Navigation Changes - e.g. Remove Tabindex and TabIndexes in v2 #3669
• Fixes ShadowStyle.Transparent is not rendering bottom-left corner correctly #3645

Proposed Changes/Todos

• Discovered serious issues with how HasFocus, OnEnter/OnLeave, etc... …work in some edge cases. This will require re-visiting the design at a deep level and fixing some long-standing but ignored issues such as how OnEnter/OnLeave don't follow proper cancelation design. Also, there's a need for keeping track of the old focus state of a tree of subviews when that tree loses focus; FocusDireciton is a hack that causes tons of confusion. See https://github.com/tig/Terminal.Gui/blob/8e70e2ae8faafab7cb8305ec6dbbd552c7bf3a43/docfx/docs/navigation.md
• Figure out how ViewArrangement.Overlapped will work. Use TabIndexes and all the View.NextView focus stuff for navigation (instead of the code in OverlappedMoveNext). Use the Subview ordering (for just the subviews with ViewArrangement.Overlapped` set) to manage the z-order.
• Rewrite focus engine
• Dozens of new, primitive unit tests
• Updated migration doc
• Massive code cleanup including making most of View #nullable enable
• View Experiments scenario -> Navigation Tester
• TabGroup navigation doesn't work completely right with nested groups. These are edge cases that I may tackle later.
• Fix TabView Scenario - above item may address
• Fix Wizards - above item may address

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tig]

@BDisp I would very much appreciate you reviewing the new Navigation.md overview document in this PR and providing feedback before I go further in implementation. You have a ton of experience with the current system for Focus and keyboard navigation.

You may also want to pull down the latest code and play with ViewExperiments. It's not fully working but is showing some progress.

Thanks.

Link here: https://github.com/tig/Terminal.Gui/blob/8e70e2ae8faafab7cb8305ec6dbbd552c7bf3a43/docfx/docs/navigation.md

[BDisp]

Change the title please, otherwise it will close the issue when merged.

[tig]

Oh. Yeah, that happens in v2_develop as well.

Do you think it's a bug on the Dim.Auto (DimAutoStyle.Content)?

We're missing SetLayoutNeeded or LayoutSubviews somewhere.

[tig]

Oh. Yeah, that happens in v2_develop as well.

Do you think it's a bug on the Dim.Auto (DimAutoStyle.Content)?

We're missing SetLayoutNeeded or LayoutSubviews somewhere.

IIRC, I think I suspected it was someting to do with SubViewsNeedsLayout not getting set right. The logic of all of that is pretty complex.

[dodexahedron]

I'm pretty tired of working on this. It was a massive effort... but well worth it IMO.

I'm bummed about what's left:

• TabGroup navigation doesn't work completely right with nested groups. These are edge cases that I may tackle later.
• Fix TabView Scenario - above item may address
• Fix Wizards - above item may address

I'm bummed because I want to work on something else for a bit.

At the same time, this is such a big PR I really want it merged.

Any objections to me leaving the above broken (I'll create Issues to track) for a bit?

Feel you, I do.

...Why does that sound so much worse than "I feel you?" I half wanted to put a 😏 on it.¹

Footnotes

1. Yeah I'm clearly 12. I mean it's right there in my name. 😅 ↩

[dodexahedron]

Oh forgot to say...

Yeah, I'll take a final pass over it after all your revisions, in case anything significant sticks out, which I'm starting as soon as I grab a source of caffeine.

Huge it definitely is, so it'll likely be a couple hours before I'm finished, simply to just read it all.

Loving the work. Focus was one of my biggest gripes from the before times, and this is a huge improvement.

[dodexahedron]

I'm not finding any show-stoppers after getting down to TabView.cs.

But it's time for me to hit the hay.

Biggest issues I do see are some pretty obvious performance issues like lines 323 through 325 of ComboBox.cs, where a collection is enumerated multiple times (up to 6 in the worst case). To be fair, that isn't newly broken, so I didn't comment on that and other similar things in-line that weren't actually changed in this PR aside from formatting.

But.

Can you at least drop a // PERF: Fix this comment or something at the top of OnHasFocusChanged on Line 308 of ComboBox.cs so it doesn't get overlooked later? That one in particular is just being needlessly cruel to the runtime.

There was one little syntax clarity tip I dropped in there, though, to demonstrate an alternative.

I'm gonna say, at this point, unless you want me to finish with the rest tomorrow, which is almost entirely test code, that it looks a'ight to me, with both the "@dodexahedron is bailing for the night" and the "@tig's bored of this one" understanding. 😅

[tig]

[tig]

Fixed some overlapped stuff while on a flight. Not really in scope for this PR, but making progress against #2491.

Lots to do to make it all work exactly right, but its promising. With far less code than was required to support overlapped in v1.

[dodexahedron]

The markdown is still broken.

Format for a URL is [text you want](url)

[v1 documentation here](This is the v2 API documentation. For v1 go here: https://gui-cs.github.io/Terminal.Gui/api/Terminal.Gui.html) is invalid markdown