Tracking multiple issues here so they don't get lost in the sea of other issues.

Ctrl+space completion (called "completion" from here on out) is plain buggy in a number of cases:

• Invoking in the middle of a symbol doesn't preselect a useful entry: Intellisense does not work well in the middle of symbol name #2200
• Completing when an underscore has been entered appends the whole token, not the remaining pieces: Underscores or special names with backticks are problematic with auto-completion #3406
• Completing when a number has been entered appends the whole token, not the remaining pieces: Completion completes too much #4515
• IntelliSense list is filtered by the characters after a number or underscore, not the whole token: Completion list appears to treat digit/underscore in identifier as a boundary #6134