Very clean. Just one change I would ask for: After each DefWndProc call, set Selectable back to true, so like this:

case InteropMisc.WM_MBUTTONDOWN:
this.SetStyle(ControlStyles.Selectable, false);
DefWndProc(ref m);
this.SetStyle(ControlStyles.Selectable, true);
break;
case InteropMisc.WM_MBUTTONUP:
this.SetStyle(ControlStyles.Selectable, false);
DefWndProc(ref m);
this.SetStyle(ControlStyles.Selectable, true);
break;
case InteropMisc.WM_MBUTTONDBLCLK:
this.SetStyle(ControlStyles.Selectable, false);
DefWndProc(ref m);
this.SetStyle(ControlStyles.Selectable, true);
break;

Doing it like this, the flicker fix still works, but it also keeps the control selectable in case someone wants to use the Tab key to select it later (if you don't set Selectable back to true, it can never be keyboard-selected after the first middle-button click).

Great work on finding a fix so fast btw. Accepting.

Once I researched how reader mode works, it was fairly straightforward to implement a simple fix. With this, I believe the RichTextBox control AngelLoader uses has been improved to a point to where no major issues remain, which is an accomplishment given how difficult it likes to be sometimes.

Thank you for considering these fixes.

No prob. You've really helped improve the experience, appreciate it.