fix(web): stop pty sizing loop on fractional cell widths

web/src/emulator/emulator.test.ts

@@ -277,6 +277,31 @@ describe('Emulator interface', () => {
     em.dispose()
   })
 
+  it('reports the exact laid-out size, not whole pixels', () => {
+    // The renderer writes the screen's true size into its style attribute,
+    // and on a Retina display that value is fractional: a cell is a whole
+    // count of device pixels, divided by the pixel ratio. offsetWidth rounds
+    // it to an integer, and that half-pixel, divided into a cell width and
+    // multiplied back out across the pane, is enough to flip cellsThatFit
+    // between N and N+1 columns depending on the dimensions the screen
+    // happens to wear — each answer reshapes the pty, the new dimensions
+    // re-round the other way, and the pane flickers with prompt redraws
+    // forever.
+    const el = document.createElement('div')
+    document.body.appendChild(el)
+    const em = createXtermEmulator({ cols: 100, rows: 24 })
+    em.attachTo(el)
+    // jsdom lays nothing out, so the renderer's write is stated by hand:
+    // 100 columns of a 7.8125px cell, 24 rows of a 17.02083…px line.
+    const screen = el.querySelector<HTMLElement>('.xterm-screen')!
+    screen.style.width = '781.25px'
+    screen.style.height = '408.5px'
+
+    expect(em.contentSize()).toEqual({ width: 781.25, height: 408.5 })
+    em.dispose()
+    el.remove()
+  })
+
   it('takes a colour palette at build time and again afterwards', () => {
     // Both matter. The option is what stops a terminal painting one frame in
     // xterm's own colours before flue's land; setTheme is what lets a running

web/src/emulator/xterm.ts

@@ -383,10 +383,24 @@ export function createXtermEmulator(opts: XtermOptions = {}): Emulator {
       if (disposed) return null
       const screen = term.element?.querySelector(SCREEN_SELECTOR)
       if (!(screen instanceof HTMLElement)) return null
-      // offsetWidth/offsetHeight rather than getBoundingClientRect, because a
-      // non-primary view is scaled by CSS: the rect would report the scaled
-      // box, and dividing the pane by that converges on nothing.
-      const size = { width: screen.offsetWidth, height: screen.offsetHeight }
+      // The style attribute, which both renderers write the screen's exact
+      // size into, rather than either measurement the browser offers. The
+      // rect is out because a non-primary view is scaled by CSS: it reports
+      // the scaled box, and dividing the pane by that converges on nothing.
+      // And offsetWidth — unscaled, the obvious next choice — rounds to
+      // whole pixels, when on a Retina display the true width is fractional:
+      // a cell is a whole count of device pixels divided by the pixel ratio.
+      // The half-pixel it drops, divided into a cell width and multiplied
+      // back out across the pane, can flip cellsThatFit between N and N+1
+      // columns depending on the dimensions the screen currently wears; the
+      // pty then answers each flip with the other one — a sizing loop that
+      // redraws the prompt several times a second for as long as the pane
+      // keeps that width. (Style, not "the obvious CSS word for it": see the
+      // Tailwind scanner note in src/styles.css.)
+      const size = {
+        width: parseFloat(screen.style.width) || screen.offsetWidth,
+        height: parseFloat(screen.style.height) || screen.offsetHeight,
+      }
       return size.width > 0 && size.height > 0 ? size : null
     },