Steps to Reproduce

1. Compile the attached solution or run compiled exe on Linux.
2. In the RichTextBox field ( AcceptsTab = true ) sequentially press: Tab, 1, 2, 3, 4 and you'll see the strange cursor behaviour ( you can see that on attached gif ).

RichTextBox_AcceptsTab_test.zip

Current Behavior

On Windows RichTextBox with AcceptsTab=true works well, but there is a bug on Linux. When you press Tab and type a text, cursor lives its own life... So it's not possible to edit text on Linux.

Expected Behavior

On Linux the same as on Windows.

On which platforms did you notice this

[ ] macOS
[* ] Linux
[ ] Windows

not tested on macOS (!)

Version Used:
Mono 32-bit 5.20.1.19

The issue tested on Ubuntu 16.4.2 desktop i386 (bug!) / Debian 9.9.0 desktop i386 (bug!) / Win7 x64 ( with Mono works well! )

The attached solution tested on SharpDevelop 4.4.2 and MonoDevelop 7.8.2 (build 2).