In Chromium browser (and Google Chrome), while trying to type in Malayalam using transliteration, Malayalam inscript or Malayalam inscript 2 keyboards available in ULS, Malayalam letters appear but revert to English on next keystroke if Wikitext syntax highlighting (CodeMirror ?) is enabled from the beta features.
In firefox, ULS keyboards work as expected but sometimes input fields freeze and no letters appear at all. But every time ULS' keyboard widget appears in an absurd position, and menu dropdown appears away from the widget (Main edit field).
Once Wikitext syntax highlighting disabled, everything goes back to normal. Please note that this happens only in 'Source editing' (ULS' inputs work pefectly in 'visual editing' mode)
Tested browsers:
Chromium Version 65.0.3325.181 (Official Build) Built on Ubuntu, running on LinuxMint Cinnamon 64 bit
Googe Chrome Version 66.0.3359.117 (Official Build) (64-bit)
{F17402173}
Firefox 59.0.2, LinuxMint Cinnamon 64 bit
{F17402196}
To recreate the problem:
1) Go to user preference, enable Wikitext syntax highlighting from beta features section
2) Go to some edit window (eg: https://ml.wikipedia.org/w/index.php?title=Test1&action=edit&redlink=1)
3) Type using ULS in visual editor's 'Source editing' mode