Examine reported "surrounding texts"

[yan12125]

Expected Behavior

Good integration with system input methods

Current Behavior

Nobody knows how it works, and sometimes relevant code causes issues. In lxqt/qterminal#806 and lxqt/qterminal#820, it is discovered that incorrect computation in TerminalDisplay::inputMethodQuery() with Qt::ImSurroundingText causes crashes under Plasma wayland [1].

[1] https://github.com/lxqt/qtermwidget/blob/master/lib/TerminalDisplay.cpp#L2910

Possible Solution

Konsole has a relevant fix [2], while it still seems wrong as it returns the whole line for "surrounding texts". Needs to investigate what "surrounding texts" actually refer to.

Some specs

• Wayland's text-input-unstable-v3 protocol [3]:

Text is UTF-8 encoded, and should include the cursor position, the complete selection and additional characters before and after them.

• GtkIMContext in GTK+ [4]:

Retrieves context around the insertion point. Input methods typically want context in order to constrain input text based on existing text; this is important for languages such as Thai where only some sequences of characters are allowed.

[2] KDE/konsole@17cb78c
[3] https://github.com/wayland-project/wayland-protocols/blob/master/unstable/text-input/text-input-unstable-v3.xml
[4] https://developer.gnome.org/gtk3/stable/GtkIMContext.html#gtk-im-context-get-surrounding

Steps to Reproduce (for bugs)

1. Create new tabs or split terminals using QTerminal under Plasma wayland

Context

Code arounding input method should really be understood and improved :)
Another long-standing issue about input methods - #127

System Information

(omitted as I cannot reproduce the bug itself. See issues linked above for reports from others)