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7a0c522a20
Note this PR should not be merged until after #600 is in. I went on a rampage tonight. It all started with wanting to use more/better characters for frame and other UI elements like the round corners:  I decided I needed a character map app that would let me test which fonts had which Unicode sets in them. As a result we have this PR - Fixes `ScrollView` in several key ways: - It now supports Computed layout and has constructors that don't require parameters. - `ScrollBarViews` are now positioned using Computed layout versus error prone absoulte - `ScrollBarViews` now correctly position themselves when one, either, or both are on/off. - `IsVertical` is now a public property that does the expected thing when changed - Mouse handling is better; there's still a bug where the mouse doesn't get grabbed by the `ScrollView` initially but I think this is a broader problem. I need @BDisp's help on this. - The `Scrolling` Scenario was enhanced to demo dynamically adding/removing horizontal/vertical scrollbars (and to prove it was working right). - I Enabled easy "infinite scroll capability" - CharMap literally lets you scroll over `int.MaxValue / 16` rows of data. Filling a `ContentView` with all of this and panning it around won't work. So I needed a way of having `Redraw` give me virtual coordinates. I did this by defining `OnDrawContent(Rect viewport)` and it's associated `event`: ```csharp /// <summary> /// Event invoked when the content area of the View is to be drawn. /// </summary> /// <remarks> /// <para> /// Will be invoked before any subviews added with <see cref="Add(View)"/> have been drawn. /// </para> /// <para> /// Rect provides the view-relative rectangle describing the currently visible viewport into the <see cref="View"/>. /// </para> /// </remarks> public event EventHandler<Rect> DrawContent; /// <summary> /// Enables overrides to draw infinitely scrolled content and/or a background behind added controls. /// </summary> /// <param name="viewport">The view-relative rectangle describing the currently visible viewport into the <see cref="View"/></param> /// <remarks> /// This method will be called before any subviews added with <see cref="Add(View)"/> have been drawn. /// </remarks> public virtual void OnDrawContent (Rect viewport) { DrawContent?.Invoke (this, viewport); } ``` I originally just implemented this pattern in `ScrollView`. Then I realized I wanted the same thing out of ALL `Views`. Namely: the ability to do drawing on an event, particularly to be able to paint something in the background. So I added it to `View`. Note, that these changes mean we are about 3 small steps away from moving the scollbars from `ScrollView` into ALL views. Which makes a lot of sense to me because I don't think we want to implement duplicative logic in, say `ListView` and `TextView` as well. Why not just do it once? Along the way I fixed some other things: - The `Checkbox.Toggled` event now passes state. Here's some gifs.  Note: - Scrollbars appear dynamically. - Fast scrolling of huge data (using no memory). - Static header - Dynamic scrollbars on/off - Note the bottom/right corner now draw correctly in all situations
