[Nawid3333]

Five small fixes in how FastStream opens the player over a page, matches
custom source patterns, and makes requests. There are seven commits, and any
of them can be dropped, except that 2 goes after 1, and 4 is three commits
that go together (reasons below). With whitespace ignored, the diff is +38/-7.

What breaks

1. A frame whose OPEN_PLAYER fails never gets a player again.
openPlayer() sets frame.playerOpening and skips any frame that has it.
The reply clears it only on 'no_video' (background.mjs:1234). When the
message fails, because the frame has no content script or content.js throws
before replying, the reply is undefined and the flag stays set. After
that, no click or new source opens a player in that frame until it loads a
new document.

2. Toolbar Off doesn't remove a player that is still opening. Off
reloads the page only if a frame has isPlayer, which PLAYER_LOADED sets.
An Off before that leaves the player on a page whose toolbar says off:

• Off while openPlayersWithSources() is still measuring the videos (an
  await on content.js): the player is opened afterwards anyway;
• Off after OPEN_PLAYER: the player iframe is already in the page, but only
  playerOpening is set, and Off doesn't look at it.

3. A frame sent to the player page plays nothing. When a video fills an
iframe that may not go fullscreen, handlePlayerOpen() navigates the frame
itself to the player (content.js:255). That fires the page's
beforeunload, which sends FRAME_REMOVED, and the background drops the
frame and its sources. The player then loads in the same frame, asks for
sources, finds none, and stays on "Welcome to FastStream".

4. Custom source patterns match the wrong type, or nothing.
MultiRegexMatcher joins the patterns into (a|b)|(c) and picks the output
from the position of the first non-empty group. Three things go wrong with
patterns a user can write:

• a. A lone type line (hls with no /regex/) becomes the empty regex.
  It matches at the start of every URL with an empty group, so match()
  returns undefined. Every other pattern with the same flags silently
  stops matching.
• b. With g, str.match() returns no groups, so the pattern never
  matches. With y, lastIndex carries over, so the pattern alternates
  between matching and not.
• c. A capture group in the user's own pattern shifts the positions:
  with m3u8 /(foo|bar)\.php/ and mp4 /baz/, a baz URL returns
  undefined. A lookahead-only pattern is lost too, since its group is ''.

5. RequestUtils.request() never settles if anything throws before
send(). The XHR is set up in an async Promise executor, so a throw from
open(), setRequestHeader() or the SET_HEADERS message only rejects a
promise nobody sees. For example, Subtitles → "From URL" accepts
https://example.com:99999/sub.vtt, the URL parser rejects it, and the
toast says "Downloading" forever.

The fix

1. The reply keeps playerOpening only for 'redirect', 'replaceall' and
   'replace', the answers that mean a player is coming. Anything else,
   including no answer, clears it.
2. Off also counts a frame with playerOpening, and
   openPlayersWithSources() returns if the tab was turned off during the
   measuring. (2 needs 1: otherwise a frame whose OPEN_PLAYER failed would
   keep the flag, and Off would reload a page with no player.)
3. handlePlayerOpen() sets RedirectingToPlayer just before the redirect,
   and beforeunload skips FRAME_REMOVED when it is set. The frame isn't
   going away: the player takes it over, and finds the frame's sources.
4. Three commits:
   • a. addRegex() refuses the empty regex, as it already refuses an
     invalid one. loadCustomPatternsFile() catches that, warns, and skips
     the line.
   • b. match() uses regex.lastIndex = 0; regex.exec(str), which is
     the same as str.match() without g/y, and correct with them.
   • c. Each output's patterns go in a named group,
     (?<__fsOutputN>...), and match() returns the output whose group
     took part (!== undefined), ignoring the user's own groups. c needs
     b, for the groups of a g regex. It also needs a: without it, an empty
     pattern would now route every URL to its type.
5. The executor's body is wrapped in try { ... } catch (e) { reject(e); }.
   The rest is only re-indented (git diff -w: +4).

Why this can't break anything else

• 1: content.js is the only handler of OPEN_PLAYER, and its real answers
  are unchanged. Only undefined (a failure) now clears the flag.
• 2: after 1, playerOpening is set only while a player is on its way,
  so reloading it away is what Off already does for a loaded player.
  openPlayer()'s own guard still stops a frame from being opened twice
  after a quick On → Off → On.
• 3: the flag is set only on the redirect branch. Every other unload
  still sends FRAME_REMOVED, and nested frames still report themselves.
• 4: patterns without g/y, without groups of their own, and not
  matching the empty string give the same results. Named groups need
  Chrome 64 / Firefox 78, and the manifests ask for 114 / 113.
• 5: the catch only runs when setup throws, before any listener can
  resolve. requestSimple() already catches a rejection and calls back with
  the error, and the other callers already handle a failed request.

Chrome: 1, 2 and 4 use the same in-memory background state as the code
around them. 3 assumes the iframe keeps its frameId when it navigates to
the extension page. I haven't checked that on Chrome. If it doesn't, the
player gets no sources, as today.

Verification

Not run in Chrome or Firefox. Node scripts ran the real code from main and
from this branch:

• for 1-3, the background's own functions and message listener, lifted as
  text, with the real TabTracker and a stubbed chrome.tabs;
• for 4 and 5, the real modules, with XMLHttpRequest stubbed for 5.

case	main	this branch
1. OPEN_PLAYER fails, then two more tries	flag stays set, 1 sent	cleared, 3 sent
2. Off while the videos are measured	player opened after Off	none
2. Off after a 'replace' answer	no reload, player stays	reload
3. sources the player gets after the redirect	[]	["https://cdn.test/v.m3u8"]
4a. hls alone + mp4 /baz/, URL …/baz	undefined	mp4 (and a warning)
4b. mp4 /baz/g, same URL twice	undefined, undefined	mp4, mp4
4b. mp4 /https:/y, same URL three times	mp4, null, mp4	mp4 ×3
4c. m3u8 /(foo|bar)\.php/ + mp4 /baz/, URL …/baz	undefined	mp4
4c. m3u8 /(?=.*master)/, URL …/master.txt	undefined	m3u8
5. open() throws	pending forever	rejects, callback gets the error
5. plain request	200	200

• Per commit: each row changes at its own commit and at no other. With
  2 applied without 1, "On, OPEN_PLAYER fails, Off" reloads a page that has
  no player.
• Build: npm ci + npm run build: all targets build, with every change
  in each zip.
• Lint: eslint at each of the seven commits: no problems.
• Merges: no conflicts with Detection: an HTML page is not a stream because its query string names one #563, which also touches background.mjs.

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