TRichView.com

Yes, RichViewTextItemClass was removed and replaced with RVItemFactory.TextItemClass.
It is mentioned in the compatibility issues for TRichView 24: https://www.trichview.com/help/new_in_version_24.html

I you want to keep compatibility with the old TRichView versions, you can use $IFDEF.
But the ...

Without StyleTemplates, Markdown will be loaded using default character attributes.
For example, *text* will be loaded as an italic text, instead of using properties of 'Emphasis' StyleTemplate.

But I think, if the style like 'Emphasis' has a font size specified, this means that the user wants to ...

If you use Win32 platform and did not change the installer option for adding path to source files for Win32 platform, RVKERNELWINVERCHECK can be defined in the project options. Page "Building | Delphi Compiler", the field "Conditional defines" is the semicolon-delimited list of compiler defines. You ...

Then I don't know why GetVersionEx behaves this way.
Try defining RVKERNELWINVERCHECK, so RtlGetVersion will be used.

This manifest must not cause problems with Win10/11 detection.
Can you double check that this manifest is really used and is not overridden by some other manifest?

Does your editor use style templates? (UseStyleTemplates property = True).
If yes, properties of certain style templates are applied on loading, such as 'Strong', 'Emphasis', 'HTML Code'. If these style templates have defined font size, it is applied to loaded text.

You can save a string to stream and call InsertMarkdownFromStreamEd.
For simplicity, you can use the RVU_UnicodeStringToUTF8Stream function from RVUni.pas (fmxRVUni.pas in FireMonkey vesion):

uses
RVUni;

procedure InsertMarkdown(const S: UnicodeString; rve: TCustomRichViewEdit);
var
Stream ...

Your application returns data corresponding to Windows 8 because its manifest does not specify compatibility with newer versions of Windows. Most likely, the <compatibility> section is missing.

The best solution is to fix the application manifest. However, you can also try recompiling TRichView ...

Integration in DeveloperExpress VCL

Changes have been made to the editors for Developer Express VCL components to allow TcxTRichViewEdit to participate in Smart Paste.

Smart Paste is an AI-powered DevExpress feature that analyzes clipboard content and automatically assigns values to appropriate ...

FireMonkey: Improvements in TRVWidget

Editing paragraph properties

The widget has been significantly improved in this version.

In the first version, its paragraph editing capabilities were limited to alignment and changing indents. In this version, it supports editing almost all paragraph ...

FireMonkey: Actions

This update adds actions .

The main differences from the VCL version (RichViewActions) are:

in VCL, actions are the basis of the user interface; in the FMX version, they are an optional addition, while the main UI is implemented using the widget;
FMX actions must be added ...

FireMonkey: TRVActionButton

The first version of FMX Tools introduced the TRVSVGPathButton component – a button with an SVG image.

This update adds another button component, TRVActionButton. It is similar to TRVSVGPathButton, but its image is selected from a predefined set using the Image ...

Improved TRVEditHelper component

TRVEditHelper is a new component for editing documents in TRichViewEdit. It is available in VCL, Lazarus, and FireMonkey. It is especially important in FireMonkey, because widgets and actions are based on it.

The TRVEditHelper component has two new methods ...

We are pleased to announce the new release of TRichView, version 24.3.

A trial version is available here: https://www.trichview.com/download/

The main new features:

improvements in TRVEditHelper component
actions for FireMonkey and a special button for them
improved widget for FireMonkey ...

The new version of TRichView editors for DevExpress VCL requires DevExpress VCL components version 25.2 or newer.

If you use an older version, comment out
{$DEFINE DX_VER_25_2}
in cxTRichView.pas and rebuild the TRichView packages (menu Start, Install TRichView VCL in Delphi IDE)