fix(clamp): measure real line height so -webkit-line-clamp works without line-height

usedLineHeightPx guessed the line box as font-size*1.2 (or the raw CSS value).
But `line-height: normal` is font-metric dependent, and inside a -webkit-box the
line box never shrinks below the font strut, so e.g. line-height:18px on 20px
text still lays out at ~20px. The guessed targetH under-counted the lines and the
clamp truncated to too few lines (or dropped the ellipsis entirely).

Measure the real per-line height from the live element (scrollHeight of a single
line) instead of guessing.

Fixes #443

Author: tinchox5

__tests__/module.lineClamp.test.js

@@ -52,6 +52,26 @@ describe('lineClamp', () => {
     expect(div.textContent).toBe(longText)
   })
 
+  it('clamps to the right line count when line-height is below the font strut (#443)', () => {
+    // In a -webkit-box the line box never shrinks below the font metrics, so a
+    // line-height smaller than the glyph height must not over-truncate. The clamp
+    // must keep as much text as a comfortably-sized line-height with the same
+    // width/font would (both fit the same 2 lines) — the old fs-based guess cut
+    // this down to a single line.
+    const make = (lineHeight) => {
+      const div = document.createElement('div')
+      div.style.cssText = `width:200px;font-size:20px;line-height:${lineHeight};word-break:break-word;display:-webkit-box;overflow:hidden;-webkit-line-clamp:2;-webkit-box-orient:vertical`
+      div.textContent = 'Long Long Long Long Long Long Long Long Long Text.'
+      document.body.appendChild(div)
+      lineClamp(div)
+      return div.textContent
+    }
+    const small = make('18px')
+    const normal = make('24px')
+    expect(small).toContain('…')
+    expect(small).toBe(normal)
+  })
+
   it('returns no-op when content fits in N lines', () => {
     const div = document.createElement('div')
     div.style.webkitLineClamp = '5'

src/modules/lineClamp.js

@@ -37,12 +37,23 @@ export function lineClamp(el) {
   if (!isPlainTextContainer(el)) return () => {}
 
   const cs = getComputedStyle(el)
-  const targetH = Math.round(usedLineHeightPx(cs) * lines + vpad(cs))
 
   const original = el.textContent ?? ''
   // Guarda para restaurar
   const prevText = original
 
+  // Measure the REAL rendered line height instead of guessing from CSS.
+  // `line-height: normal` is font-metric dependent, and inside a -webkit-box the
+  // line box never shrinks below the font strut, so a value smaller than the
+  // glyph height (e.g. line-height:18px on 20px text) still lays out taller. A
+  // fs*1.2 / raw-CSS guess mis-sizes targetH and clamps to the wrong line count (#443).
+  const pad = vpad(cs)
+  el.textContent = 'X'
+  const perLine = el.scrollHeight - pad
+  el.textContent = original
+  const lineH = perLine > 0 ? perLine : usedLineHeightPx(cs)
+  const targetH = Math.round(lineH * lines + pad)
+
   // Si ya entra completo en N líneas, no hacemos nada (igual que el clamp nativo)
   if (el.scrollHeight <= targetH + 0.5) {
     return () => {}