Missing characters in TextInput field from fast sources (e.g. barcode scanners)

[cstatz]

Software Versions

• Python: 3.7.3
• OS: Raspbian Buster
• Kivy: 2.0.0rc1, sdl2 backend
• Kivy installation method: python3 setup.py develop --user

Describe the bug
When using fast keyboard-like devices (e.g. barcode scanners) characters are lost in the TextInput field (for longish barcodes). For example: 1234ABCDE12345678 could end up as 12DE12345 or 12DE!"§$%.
If the characters end up upper case the "virtual caps-lock" remains until the restart of the kivy app. I was able to observe this behavior with multiple apps using TextInput fields on raspbian with the sdl2 backend.

Expected behavior
I would expect the text is correctly captured independent of input device and speed. No characters or symbols are missing and there should be no "virtual caps-lock" when characters go missing.

To Reproduce
Use a barcode scanner (without any interkey delay - this "fixes" the problem, but is a nuisance for long barcodes in productive settings) with the Kivy RST-Editor example. Continuously scan long barcodes (>10 symbols) with letters (upper/lower case) and numbers.

Code and Logs and screenshots
There was no obvious error related to this issue in the logs.

Additional context
I was able to narrow down the problem to the kivy/core/window/_window_sdl2.pyx.
In _WindowSDL2Storage:

• (line 536, class member poll) SDL_PollEvent is called with the gil released. The return value rv is hence in an undefined state after the with nogil block.
• (line 530, class member wait_event) SDL_WaitEvent(NULL) is called with gil released and never waited.

Removing the nogil statements (around the aforementioned lines) fixes my barcode scanner problem (but may cause other problems).

[Julian-O]

Sounds awfully like #8061 could be related