Fixes#7536
When we're reflowing a row and we need to insert a spacer head, we must
move to the next row to insert it. Previously, we were setting a spacer
head and then copying data into that spacer head, which could lead to
corrupt data and an eventual crash.
In debug builds this triggers assertion failures but in release builds
this would lead to silent corruption and a crash later on.
The unit test shows the issue clearly but effectively you need a
multi-codepoint grapheme such as `👨👨👦👦` to wrap across a row by changing
the columns.
aab00da242