Autosizing on {NS,UI}Textview works, but background colours leak outside the view

All,

I am programmatically creating NSTextView (and UITextView on iOS) objects on a canvas (just an {NS,UI}View. I am working on auto-resizing (which works), but there are some strange behaviours.

Here is what I want (and what I 95% achieve):

• create textview at cursor position (its actually a subclass of NS/UI TextView)
• have a border around textview, which follows the textview size. This includes a grab bar header for moving and a grab region for manual sizing (the double headed arrow icon). I do this with a layer that uses constraints to follow the text view size.
• textview initially starts at a small size (50px wide by about 2 lines tall).
• textview grows wider as you type up to a maximum width, where it stops growing horizontally and starts growing vertically.
• support a variety of pre-defined styles.
• allow the textview to be manually sized, after which its width is fixed (but keeps growing vertically as required).
• Allow the text to be grabbed/moved by the grab bar header

This all works (creating, moving, autosizing, styles, borders). For example, here is an initial text view just after creation.

Here is some text being added and autosizing happening.

The Problem:

Where it gets weird is when I apply styles with a background color - then the TextContainers width shows up. Here is what it looks like when I apply a style with a background color.

I expected the NSTextView to crop the excess TextContent, but it does not.

Extra information:

The trick to make autosizing work is to set the TextContainer size to a large width, and turn off widthTracksTextView. This approach is outlined here: How to resize a NSTextView automatically as per its content? (complete with a video).

Here is the sizing initialization code from my NSTextView subclass:

VerticallyResizable = true;
HorizontallyResizable = true;
TextContainer.WidthTracksTextView = false;
TextContainer.HeightTracksTextView = false;
var s = TextContainer.Size;
s.Width = 1000;
TextContainer.Size = s;
MaxSize = new CGSize(1000, 10000);

Does anyone have any thoughts as to what I’m missing? Thx

Addendum

As far as I can tell, these are the best approaches:

• Fix the width. This way TextContainer and NS/UI TextView will have the same width always and there will be no drawing of paragraph styles beyond the TextView frame. Downside - no auto-sizing horizontally.
• Have the TextContainer width follow the textview (rather than the other way around). I've been experimenting with this, but because the TextContainer is now fixed, I can't get the new required width after text is inserted (i.e. you want to call LayoutManager.EnsureLayoutForTextContainer then GetUsedRect, but this returns the current width).

The solution as provided by Willeke was to simply ensure that ClipsToBounds is set, and that it stays set. For some reason it gets reset after the constructor completes.

Interestingly this is not needed on iOS which seems to have clip set to true.