The dialog box for inclusion of tags is very weak. Page down key does not work. Scroll bar is narrow. There is no editing of tags for misspellings. It even seems to suggest tags that you chose not to include the last time you created a catalog.
You are describing the Text input dialog provided to assign keywords to selected files in the browsing space. It's a wonderful tool which lets me tag my pictures much, much quicker than with previous versions. I simply enter the first characters of the keyword, and I am instantly provided with a reduced list of keywords from my personal list (more than one thousand...) Generally, the first three characters are enough. But... The problem is that it's a too sophisticated text input dialog: besides its primary text input function, it also can browse an alphabetical list of keywords in that small area with a small scrollbar. I am sure that not even one user in a thousand use that function.
Edit: By the way, the normal tool to select by keywords is the 'Keywords' tab in the top of the right panel. The same tool is used to select, but also to assign keywords.