"Flexible Mode" loses insertion point when resizing window

When the window is resized, `relayoutTypewriterMode(scrollView:)` is called to make the active typewriter mode object recalculate the necessary `textContainerInsets`. By increasing the top inset to always equal `100% of the container view - lineHeight`, this mode effectively pushes the insertion point out at the bottom.

Start at relayoutTypewriterMode(scrollView:) and trace how resizing recalculates textContainerInsets for the active typewriter mode object. Reproduce the window-resize behavior in the macOS text view and inspect where the insertion point moves. Done means the insertion point remains visible and correctly positioned after resizing in Flexible Mode.

Written by the indexing model from the issue text.