Fix: caret can fall off-screen after typing on long lines

[andrewtheart]

What

TextActionManager.AddCharacter was the only edit operation still using the line-granularity ScrollManager.ScrollLineIntoViewIfOutside(...) to keep the caret visible after an edit. Every other edit operation uses ScrollManager.UpdateScrollToShowCursor(...):

Edit op	Scroll call
Backspace / Delete / AddNewLine / Undo / Redo	UpdateScrollToShowCursor
AddCharacter (typing)	ScrollLineIntoViewIfOutside ← inconsistent

Why it matters

ScrollLineIntoViewIfOutside only scrolls when the caret's line is outside the viewport. While typing on a single long line, the line is always "in view", so it never scrolls to follow the caret horizontally — the caret (and the next typed character) can end up past the right edge, outside the rendered area. The other edit operations don't have this problem because they use the cursor-aware helper.

Change

One line in AddCharacter: use the same UpdateScrollToShowCursor(false) helper the rest of the edit pipeline already uses, so caret-follow after typing is consistent with backspace/delete/newline/undo/redo.

Testing

• Builds clean (TextControlBox.csproj, Release/x64).
• Behavior: typing past the right edge of a long line now keeps the caret in view, matching the other edit operations.

This fix was developed while using TextControlBox as a vendored editor; contributing it back upstream.