fix(markup): correct Firefox caret height near placeholder

[anademi]

Fixes #1222

Summary by Sourcery

Bug Fixes:

• Correct the Firefox caret height and alignment when it appears adjacent to a placeholder.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

The PR updates the CodeMirror gravity theme with a Firefox-only CSS rule that baseline-aligns a .cm-widgetBuffer adjacent to a .cm-placeholder, correcting the caret height when the caret is near placeholder content without affecting other browsers.

File-Level Changes

Change	Details	Files
Add a Firefox-specific caret alignment workaround for placeholders.	Detect Firefox using a vendor-specific feature query. Align a widget buffer immediately preceding a placeholder to the text baseline. Scope the workaround to the existing CodeMirror gravity theme.	packages/editor/src/markup/codemirror/gravity.ts

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#1222	Ensure the caret next to the markup placeholder has the correct editor-line height in Firefox, both before and after the placeholder is clicked.	✅

Possibly linked issues

• Incorrect caret height next to the markup placeholder in Firefox #1222: The PR applies a Firefox-specific baseline alignment fix for the widget buffer beside the placeholder.

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[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="packages/editor/src/markup/codemirror/gravity.ts" line_range="36-40" />
<code_context>
     '.cm-placeholder': {
         color: 'var(--g-color-text-secondary)',
     },
+    '@supports (-moz-appearance: none)': {
+        '.cm-widgetBuffer:has(+ .cm-placeholder)': {
+            verticalAlign: 'baseline',
+        },
+    },
     '.cm-content': {
</code_context>
<issue_to_address>
**issue (bug_risk):** Firefox versions before 121 accept the `-moz-appearance` feature test but do not support `:has()`, so the nested rule is discarded and the caret retains the incorrect height/alignment in those browsers.

**Triggers:** When the editor runs in Firefox 120 or older, including Firefox ESR 115.

**Suggested fix:** Use a selector supported by the minimum Firefox version, or add a fallback rule that does not depend on `:has()`.
</issue_to_address>

Sourcery assessment

Approval pending. 1 finding to address first.

Blocking findings: packages/editor/src/markup/codemirror/gravity.ts:40

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[gravity-ui]