Review of the previous commit found that waiting for the font does not, on its
own, do anything.
`FitAddon.proposeDimensions()` measures nothing — it divides the container by a
CACHED cell size, and xterm refreshes that cache only from `open()`, from a
resize that actually changed the grid, and on a device-pixel-ratio change.
Nothing in it listens for font loading. So a fit that runs after the font
arrives can still divide by the fallback cell, propose the grid it already has,
and short-circuit before anything re-measures. The wait now ends by calling
`_charSizeService.measure()` itself, which is the step that makes the following
fit see the real font. Private API, as FitAddon's own dependency on `_core` is,
and guarded because a terminal can be disposed mid-wait.
The wait was also unbounded, and it sat behind the buffer-load gate. Neither
`FontFaceSet.load()` nor `FontFaceSet.ready` has a deadline, so a font request
that never settled left the tab spinning with live output queued behind it —
permanently, and on every session, since they share one promise. The comment
claimed the opposite ("a font that never loads must not block the terminal, so
this always resolves"), which was true of the per-face loads and false of
`ready`. It is now raced against TERMINAL_FONT_WAIT_MS, and the await moved
ahead of `_beginBufferLoad` so a slow font cannot hold output back at all —
which also removes the stale-select interaction with `_restoringFlushedState`,
since that flag is not yet set when the wait runs.
The awaited set no longer includes faces that cannot move the measured cell.
The bundled symbols font is ~1.2MB of private-use-area glyphs and xterm
measures `W`, so awaiting it put a megabyte in front of the first frame for
nothing; the generic families match no FontFace at all.
A runtime font change had the same race the boot-time one did:
applyTerminalFontFamily wrote the new family and fit on the next line, against
a family the browser might not have loaded. It now re-arms the wait and fits
again when it settles.
The claim that this could not be tested was wrong: the repo's vm harness
reaches both halves. The new suite pins the family filter, the forced
re-measure, the deadline, a rejecting load, a browser with no font API, and a
terminal disposed mid-wait — plus the four ordering properties in
selectSession, including that iOS Safari's synchronous focus still precedes the
first await. Each assertion was checked by reverting its fix.
Also corrects the docstring's reason for calling `document.fonts.load` (the
stylesheet is render-blocking and long parsed by then; the real reason is that
the WebGL renderer rasterises through a canvas atlas, and canvas text never
triggers a CSS font fetch), restores the JSDoc block the previous commit
displaced from getTerminalDimensions, and fixes a comment that described the
first fit as already having run when the mobile-Safari branch defers it.
