Add font-specific alignment customizations for Label rendering

[fmoo]

Describe the project you are working on

A 2D game utilizing pixel fonts with latin-1 glyphs.

Describe the problem or limitation you are having in your project

When designing and laying out text within a panel UI, content is perceived as aligned when the margins are evaluated against the the box consisting of cap-height <-> baseline.

When using latin-1 pixel fonts, ascenders on accented capital letters generally exceed the cap-height. Godot seems to size and align labels according to the maximum glyph sizes, which results in text rendered inside of a margins being applied in a way that appears asymmetric, like this:

One might hack around this by simply reducing the vertical margin, or setting a margin top constant override for a font-specific Theme, but this problematic for a few reasons:

• this requires you to set different panel margins differently based on your font, instead of being based on the panel texture
• if a rich text label ever needs to wrap, the scrollbar will spill up into the panel area:

Describe the feature / enhancement and how it helps to overcome the problem or limitation

I'd propose one or more of the following:

• A boolean theme override or flag constant on text controls, "align top to cap height" that causes the text to be aligned and sized according to the cap-height of the font instead of the total height including all ascenders
  • Similarly, one for "align bottom to baseline" so bottom-aligned text can be aligned according to the baseline instead of according to the total depth including all descenders on all glyphs
• A numeric theme override on text controls "ascender offset" that can be used to adjust how to align top-aligned text within a margin
  • Similarly, a numeric theme override for "descender offset" that can be used to adjust how to align bottom-aligned text within a margin

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

New theme constants or label attributes should be modifiable via the Inspector panel when viewing the Theme, Label, and RichTextLabel as appropriate.

If this enhancement will not be used often, can it be worked around with a few lines of script?

No workarounds via script that can address all of the issues are known at this time:

• Adjusting the positioning of the vscrollbar when a RichTextLabel collapses
• Generic support for a panel to support all fonts without needing per-font margin properties, Themes, and/or Theme Overrides for on MarginContainer.

Is there a reason why this should be core and not an add-on in the asset library?

The affected controls (Label, RichTextLabel) are part of core.

[fmoo]

Was talking about this issue with some more people this morning, and they pointed out that using a FontVariant instead of a FontFile provides a way to accomplish exactly this via negative Extra Spacing > Top and Extra Spacing > Bottom values:

I do think it's possibly worth exposing an override for these directly on the theme or text controls, or trying to improve user awareness of the available features on FontVariant, but as there's a workable solution for this class of issue, this proposal is not going to be as impactful.

[bruvzg]

baseline offset is already available in 4.3 (as a FontVariation property).

align top to cap height

For most fonts, you can't easily determine cap height, so this won't work. Aligning baseline to the bottom can be added (but I'm not sure if it's something that will be used a lot).