If you don't like new dark popup window with letter - you can change it in Settings - Appearance - last section "Show keys preview"

Jbak keyboard contains all you need for comfortable text input.
Keyboard is very small but has many functions and settings. Here you can find many keyboard layouts for more than 60 languages. For 30 languages you can download dictionary and it gives additional features: text input using gestures word corrections.
Voice text input offers several results and supports manual input of punctuation marks - so you can dictate large texts without errors.
With help of special tables you can enter over 60000 symbols and over 1000 emoticons. In Pro-version you can create your own sets of symbols and emoticons.
Keyboard also contains list of texts from clipboard for quick pasting into any text field.
After entering password in any text box you can hit "Save" on top panel - and you can enter this password later in one click. In free version you can save only two passwords Pro-version has no restrictions.
After installation you need to run application. Press first menu item - to activate keyboard. Then press first menu item again to select active keyboard. Also here you can change design size of keyboard keyboard layout sound vibro and much more.