`FillRect` Corrupts Wide Characters When Overlapping

[tig]

Original Issue:

When OutputBufferImpl.FillRect() overwrites cells containing wide characters (e.g., Chinese text), it directly sets cell contents without properly handling wide character invalidation. This causes visual corruption when UI elements like MessageBox borders are drawn over wide character content.

Reproduction

1. Display Chinese text (e.g., "你好呀") in a view
2. Draw a MessageBox that overlaps the text
3. Move mouse over the MessageBox border (triggers HighlightState changes and redraws)
4. Observe corrupted rendering where wide characters are partially overwritten

See the ChineseUI scenario in UICatalog for a concrete example.

Root Cause

Without Fix:

• FillRect() directly assigns cell values without calling AddStr()
• Wide characters that get partially overwritten are not invalidated
• Adjacent cells are not marked dirty
• The second column of a wide character retains stale data

In OutputBufferImpl.FillRect():

#else
Contents [r, c] = new ()
{
    Grapheme = rune != default (Rune) ? rune.ToString () : " ",
    Attribute = CurrentAttribute, 
    IsDirty = true
};
#endif

[tig]

If the two places in View.Mouse.cs where we do this:

        if (HighlightStates != MouseState.None)
        {
            SetNeedsDraw ();
        }

Are changed such that SetNeedsDraw is not called, the bug is not repo. Of course, that also means that highlightstates stops working.

When LayoutAndDraw is called after these SetNeedsDraw calls, the code in Border (and Margin) that deal with HighLightStates, along with this code in GetAttributeForRole:

        if (role != VisualRole.Disabled && HighlightStates != MouseState.None)
        {
            // The default behavior for HighlightStates of MouseState.Over is to use the Highlight role
            if (((HighlightStates.HasFlag (MouseState.In) && MouseState.HasFlag (MouseState.In))
                 || (HighlightStates.HasFlag (MouseState.Pressed) && MouseState.HasFlag (MouseState.Pressed)))
                 && role != VisualRole.Highlight && !HasFocus)
            {
                schemeAttribute = GetAttributeForRole (VisualRole.Highlight);
            }
        }

...cause things to be redrawn, but the only thing that actuallyi gets changed are attributes.

This points to another bug in either AddString or OutputImpl.Write regarding writing Cells where only the attribute has changed.

[BDisp]

...cause things to be redrawn, but the only thing that actuallyi gets changed are attributes.

This points to another bug in either AddString or OutputImpl.Write regarding writing Cells where only the attribute has changed.

Any change to an attribute logically implies a complete redraw of the cell.

[BDisp]

I'm posting this here because I'm not sure if it's related to the same problem. Glyphs disappear when moving the Dialog. If it is not the same problem, please create a new issue.