Non-API breaking change notes for changing the default test font

src/release/breaking-changes/rendering-changes.md

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+---
+title: Notable Rendering and Layout Changes after v3.7
+description: Non-API related breaking changes made after Flutter v3.7.
+---
+
+## Changes
+
+This section lists the notable non-API breaking changes.
+
+### (Only Affects Tests) `FlutterTest` is now the default test font
+
+The `FlutterTest` font replaced `Ahem` as the default font in tests: when 
+`fontFamily` isn't specfied, or the font families specified are not registered, 
+tests use the `FlutterTest` font to render text. The `Ahem` font is still 
+available in tests if specified as the `fontFamily` to use.
+
+The `FlutterTest` font produces more precise font and glyph metrics than `Ahem`, 
+and the metrics are generally font-engine agnostic. See the [Flutter Test Fonts][] 
+wiki page for more details about the test font.
+
+**Differences**
+
+The `FlutterTest` font looks almost identical to the old default `Ahem`: the
+glyph for most characters is a box that fills the em square. The notable
+differences between the `FlutterTest` font and `Ahem` font are:
+
+**1. Different Baseline Location**
+
+The `FlutterTest` font's ascent and descent are 0.75 em and 0.25 em, while
+`Ahem`'s are 0.8 em and 0.2 em respectively.
+
+In the example golden image change below, the white blocks are text rendered
+using `Ahem` and `FlutterTest`. The second character is taller in the new font
+since it has a larger descent.
+| Before (`Ahem`) | After | Animated Diff |
+| :---: | :---: | :---: |
+| ![before](assets/material.ink_sparkle.bottom_right.0_masterImage.png) | ![after](assets/material.ink_sparkle.bottom_right.0_testImage.png) | ![baseline_animated](assets/baseline.gif) |
+
+**2. Different Decoration Position**
+
+The underline location is slightly higher in `FlutterTest` than `Ahem`.
+
+In the example golden image change below, the 3 lines of white blocks are text 
+rendered using `Ahem` and `FlutterTest`. The blue dashed lines indicate the 
+[TextDecoration.overline]/[TextDecoration.lineThrough]/[TextDecoration.underline] 
+positions for each line.
+| Before (`Ahem`) | After | Animated Diff |
+| :---: | :---: | :---: |
+| ![before](assets/widgets.text_golden.Decoration.1_masterImage.png) | ![after](assets/widgets.text_golden.Decoration.1_testImage.png) |  ![baseline_animated](assets/underline.gif) |
+
+**3. The glyph used for unmapped characters are slightly different**
+
+Unmapped characters are rendered as hollow boxes in both fonts, with a slight
+difference:
+| Before (`Ahem`) | After | Diff |
+| :---: | :---: | :---: |
+| ![before](assets/material.floating_action_button_test.clip_masterImage.png) | ![after](assets/material.floating_action_button_test.clip_testImage.png) | ![not_def_animated](assets/not_def.gif) |
+
+**References**
+
+Relevant PRs:
+* the `FlutterTest` font was added in: [Add new test font]({{site.repo.engine}}/pull/39809)
+* the `FlutterTest` font was made the default in: [Make FlutterTest the default test font]({{site.repo.engine}}/pull/40188)
+
+Wiki Page:
+* [Flutter Test Fonts][]
+
+[Flutter Test Fonts]: {{site.repo.flutter}}/wiki/Flutter-Test-Fonts
+[TextDecoration.underline]: {{site.api}}/flutter/dart-ui/TextDecoration/underline-constant.html
+[TextDecoration.overline]: {{site.api}}/flutter/dart-ui/TextDecoration/overline-constant.html
+[TextDecoration.lineThrough]: {{site.api}}/flutter/dart-ui/TextDecoration/lineThrough-constant.html