Adding AppContext Switch for disabling special character ligatures

[Kuldeep-MS]

Contributes to #109

Description

Special character ligatures will be included in .NET 9 preview 4. This PR is introducing an AppContext switch to disable the feature.

Customer Impact

Without this changes customer won't be able to disable the special character ligature.

Regression

NO

Testing

DRT + Microsuites + Feature Tests

Risk

Low. After upgrading to .NET 9 preview 4, WPF applications which have editing features that use special character ligatures may notice some unforeseen issues. For such cases, an AppContext switch (Switch.System.Windows.DisableSpecialCharacterLigature) can be set to true which will disable the feature.

[miloush]

I will just note as I already stated elsewhere that it is not sustainable long-term to gate every change to WPF by an AppContext switch. Other avenues need to be considered, or a plan for removing them in the future should be in place (e.g. every other LTS release).

The risk associated with the changes should play a role. Compared to .NET Framework, there is an abundance of releases and if a change breaks someone, they can easily stay on a previous release and raise the issue, and switch can be added as a part of maintenance update if justified.

For this one, like in the case of hardware acceleration for RDP, the cost of the introduced complexity, maintaining and documenting the switch is not justified by the low risk of the change in my opinion.

[Kuldeep-MS]

I will just note as I already stated elsewhere that it is not sustainable long-term to gate every change to WPF by an AppContext switch. Other avenues need to be considered, or a plan for removing them in the future should be in place (e.g. every other LTS release).

The risk associated with the changes should play a role. Compared to .NET Framework, there is an abundance of releases and if a change breaks someone, they can easily stay on a previous release and raise the issue, and switch can be added as a part of maintenance update if justified.

For this one, like in the case of hardware acceleration for RDP, the cost of the introduced complexity, maintaining and documenting the switch is not justified by the low risk of the change in my opinion.

I completely understand and appreciate your suggestion. However, for this particular change, which is a low level change, we have planned initially, we would like to incorporate an AppContext switch to mitigate any unforeseen issues. As we have faced similar situations in the past, we want to take extra precautions by implementing this switch.

We will discuss more on this internally and if there is a need to revisit to remove switches, we will definitely do it.