Spurious characters on compose sequence input in gtk2

Original Reporter info from Mantis: munizao

• Reporter name: Alexandre Muñiz

Description:

When I type in characters using a compose sequence, extra spurious characters are inserted. For example, when I type:
[compose] n
I get:
�ñ
(That's a 000E character before the 'ñ'.) The extra character comes when I type the n, then the 'ñ' is input as normal when I type the ''.
When I type:
[compose] x x
I get:
��×
(Those are two 0018 characters before the '×'.) In this case, I get the first spurious character when I type the first x, and the second spurious character and the correct character together when I type the second x.

I believe I am using the default XIM input method.

I previously reported this as a bug in Kmemo, but they claimed the bug was in Lazarus LCL itself. See this KControls issue for more information: https://github.com/kryslt/KControls/issues/31

Steps to reproduce:

Set up a compose key in Keyboard Preferences → Layouts → Options.
Type [compose], then 'n', then '~'

Mantis conversion info:

• Mantis ID: 38454
• OS: Linux Mint
• OS Build: 20
• Platform: Linux
• Version: 2.0.6
• Monitored by: » AntonK (Anton Kavalenka), » @Alexey-T1 (CudaText man), » @davidbannon (David)