fixes #20622

fixes #20601

fixes #20603

Part of #18454

Fixes #19026

peekLF and peekSingleByteChar apparently aren't inlined, hurting the perf in this very hot loop. The perf regression is partly from function calls, and partly from the function depending on a closure. It also seems like there might be more gains to be had if I refactor this file to not depend so heavily on closures and nested anonymous functions.

fixes #18672

Colour for column selection in debugging mode added to high contrast theme. Amended yellow frame column selection to stand out in light themes.

fixes #20567

takes up too much vertical space as is. Also change the string to make it less likely to wrap and take another line.