Fix Ctrl+Backspace affecting ReadOnly TextBox

[NikolayXHD]

Fixes #6667 introduced by #6256

Test methodology

Manually verified the problem from screenshot below was fixed.

Test environment(s)

• Windows 7 SP1

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✒️ I contribute this code under The Developer Certificate of Origin.

[codecov]

Codecov Report

Merging #6668 into master will decrease coverage by 0.11%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #6668      +/-   ##
==========================================
- Coverage   47.35%   47.23%   -0.12%     
==========================================
  Files         704      704              
  Lines       52779    52778       -1     
  Branches     6952     6953       +1     
==========================================
- Hits        24993    24930      -63     
- Misses      26444    26528      +84     
+ Partials     1342     1320      -22

Flag	Coverage Δ
#production	36.48% <0%> (-0.15%)	⬇️
#tests	97.61% <ø> (ø)	⬆️

[RussKie]

Thank you.

Btw have you seen how the similar behaviour was implemented in dotnet/winforms#629?

[NikolayXHD]

Thank you.

Btw have you seen how the similar behaviour was implemented in dotnet/winforms#629?

Thanks for pointing me to it. Looking at it I found a similar case I missed here: ComboBox when DropDownStyle == ComboBoxStyle.DropDownList should not be affected by hotkey as well.