Inconsistent behavior when trying to highlight C # TextBox

Question

I am creating a C # WinForms program, and my text fields do not allow the user to sequentially select text throughout the program.

In some places, the backlight works fine: you enter something into the field, click and drag some text, and also select where you dragged.

Elsewhere, clicking and dragging does not select text. The only way to do this is to double-click on the text.

I have not changed any default properties of these text fields or confused with any event listeners. I placed new text fields in different places and they behave differently.

I am wondering if this is due to the properties of the form in which it is located TextBox, because it seems that either all the text fields in a particular form work or do nothing. However, as far as I can tell, the properties look the same in all directions, and I never remember changing anything.

It seems to me that this happens randomly. I can not find any information on this topic. Does anyone know what I'm talking about?

EDIT: Well, I figured out where the problem is, but I still don't know how to fix it.

This only happens on forms that have been added to SplitContainerin my main window as follows:

myForm.TopLevel = false;
this.splitContainer.Panel2.Controls.Add(myForm);
myForm.Show();

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Answer 1

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MyForm frm = new MyForm();
frm.MdiParent = this;
frm.Show();

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Answer 2

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