Intent to ship: Canvas2d TextMetrics.*Baseline attributes

[Jonathan Kew]

As of Firefox 116, I intend to turn the baseline attributes[*] of the
Canvas2D TextMetrics interface on by default.

This was developed behind the dom.textMetrics.baselines.enabled preference.

Status in other browsers is:
Chrome - not yet implemented (they have an older non-standard
baselines() function that addresses similar functionality)
Safari - shipping, though results are not always accurate

Bug to turn on by default:
https://bugzilla.mozilla.org/show_bug.cgi?id=1840075

Standard: https://html.spec.whatwg.org/multipage/canvas.html#textmetrics

These attributes were originally implemented behind a pref in bug
1102584, but used fallback baseline heuristics rather than reading real
baseline data from the font, and were not shipped due to concerns over
the interoperability of the resulting values.

In bug 1840075, I'm updating the implementation to correctly respect the
actual baseline information in the font (when present), and therefore we
can expose the attributes as they will be returning accurate values.

JK

[*] This covers three individual TextMetrics attributes:
alphabeticBaseline
hangingBaseline
ideographicBaseline


- - -
Original "Intent to Prototype" email, from 2020-01-17 (as I can't find
it in the Google Groups archive to link to...)
- - -

The TextMetrics interface represents the dimensions of a piece of text
in the canvas, as created by the CanvasRenderingContext2D.measureText()
method.

Currently, Gecko only supports the .width attribute, but authors would
also like to determine the actual dimensions that the rendered text
would have in both x- and y-directions, as well as overall font metrics
to assist with line spacing, etc.

Safari supports the full set of TextMetrics attributes as currently
documented in the HTML spec. Chrome has recently shipped support for the
actualBoundingBox attributes; it has support for additional font-metrics
attributes behind the "experimental web features" flag.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1102584

Standard: https://html.spec.whatwg.org/multipage/canvas.html#textmetrics

Note that there are a number of open issues regarding the spec (see
https://github.com/w3ctag/design-reviews/issues/302#issuecomment-427996200).
For this reason, I propose to implement these attributes behind several
prefs, and initially enable only the actualBoundingBox* attributes by
default (matching what Chrome has shipped); the others will be
preffed-off, but can be selectively enabled by authors who want to
experiment with them.

Testing: Some (limited) tests are in
web-platform/tests/2dcontext/drawing-text-to-the-canvas/2d.text.measure.*

Platform coverage: All

Restricted to secure contexts: No. The existing TextMetrics interface is
not restricted, and other browsers support the extended attributes in
all contexts.

Target Release: 74

Preferences behind which this will be implemented:
dom.textMetrics.actualBoundingBox.enabled (default: true)
dom.textMetrics.baselines.enabled (default: false)
dom.textMetrics.emHeight.enabled (default: false)
dom.textMetrics.fontBoundingBox.enabled (default: false)