This vid demonstrates non-rectangular clip regions doing their magic!

Loving the work on this. 👍

Planning to take a good look at the code later today.

The current state is pretty good with the exception of:

• Shadows (actually Margin in general) are broken
• ScrollView is broken (and I won't fix... we have a new one coming)
• TabView is broken. Bdisp has a pr i haven't had a chance to look at.
• Attribute setting is brute force. Need to really clean that up because Color comparison is a heavy op.

I have not invested as much energy in unit tests as I've liked.

This is how this all works (from drawing.md):

Drawing occurs each MainLoop Iteration

The @Terminal.Gui.Application MainLoop will iterate over all Views in the view hierarchy looking for views have their @Terminal.Gui.View.NeedsDraw property set. The @Terminal.Gui.View.Draw method will be called which, in turn:

1. Sets the clip to the view's Frame.
2. Draws the @Terminal.Gui.View.Border and @Terminal.Gui.View.Padding (but NOT the Margin).
3. Sets the clip to the view's Viewport.
4. Sets the Normal color scheme.
5. Calls Draw on any @Terminal.Gui.View.Subviews.
6. Draws @Terminal.Gui.View.Text.
7. Draws any non-text content (the base View does nothing.)
8. Sets the clip back to the view's Frame.
9. Draws @Terminal.Gui.View.LineCanvas (which may have been added to by any of the steps above).
10. Draws the @Terminal.Gui.View.Border and @Terminal.Gui.View.Padding Subviews (just the subviews). (but NOT the Margin).
11. The Clip at this point excludes all Subviews NOT INCLUDING their Margins. This clip is cached so @Terminal.Gui.View.Margin can be rendered later.
12. DrawComplete is raised.
13. The current View's Frame NOT INCLUDING the Margin is excluded from the current Clip region.

Most of the steps above can be overridden by developers using the standard Terminal.Gui cancellable event pattern. For example, the base @Terminal.Gui.View always clears the viewport. To override this, a subclass can override @Terminal.Gui.View.OnClearingViewport to simply return true. Or, a user of View can subscribe to the @Terminal.Gui.View.ClearingViewport event and set the Cancel argument to true.

Then, after the above steps have completed, the Mainloop will iterate through all views in the view hierarchy again, this time calling Draw on any @Terminal.Gui.View.Margin objects, using the cached Clip region mentioned above. This enables Margin to be transparent.

Declaring that drawing is needed

If a View need to redraw because something changed within it's Content Area it can call @Terminal.Gui.View.SetNeedsDraw. If a View needs to be redrawn because something has changed the size of the Viewport, it can call @Terminal.Gui.View.SetNeedsLayout.

Clipping

Clipping enables better performance by ensuring on regions of the terminal that need to be drawn actually get drawn by the @Terminal.Gui.ConsoleDriver. Terminal.Gui supports non-rectangular clip regions with @Terminal.Gui.Region. @Terminal.Gui.ConsoleDriver.Clip is the application managed clip region and is managed by @Terminal.Gui.Application. Developers cannot change this directly, but can use @Terminal.Gui.Application.ClipToScreen, @Terminal.Gui.Application.SetClip(Region), @Terminal.Gui.View.ClipToFrame, and @Terminal.Gui.ClipToViewPort.

I am now working through what it would mean to have Border be treated like Margin above: Be excluded from the Clip of a View and rendered in a 2nd pass. This should make the whole LineCanvas thing work much more cleanly.

This is now ready for real review.

Still some to do, but mostly unit tests.