Prevent a second download starting when Continue is pressed twice #112

After pressing Continue, the Main dialog closes, the pre-download checks run, and then the Main dialog is redisplayed with the URL still in the text box. During that gap nothing indicates work is happening, so it reads as a click that didn't register — and Continue is sitting right there to press again. Doing that starts a second download of the same URL.

When that happens, two yt-dlp processes work against the same output file names. In my case the log ended after the [download] Destination: lines with no [Merger] step, and the file left behind was the bare video stream renamed .mp4 — AV1, no audio track, and the "QT compatible video" re-encode never ran despite being enabled. That file plays on a recent Apple-silicon Mac and fails on older ones, so it looks like a codec bug rather than a duplicate run.

Two suggestions, either or both:

1. Guard against resubmission

When the Main dialog is redisplayed after a download has been launched, ignore Continue for a URL already in progress (or clear the field, as the existing "Discard URL after download" setting does).

2. Show progress during the checks

An indeterminate indicator using AppleScript's built-in progress properties — available from 10.10, so no change to the minimum OS. There is already a "Please wait" alerter notification for the channel check around line 1802; this would cover the ordinary single-video path, with no extra process to kill on the error paths.

Suggested code for (2), in Code/Main.applescript:

Add two handlers:

on show_busy(theText)
	-- Show an indeterminate progress indicator while pre-download checks run.
	-- These checks can take several seconds with no feedback on screen, so users press
	-- Continue a second time and two downloads start on the same file names.
	set progress total steps to -1
	set progress completed steps to 0
	set progress description to theText
	set progress additional description to ""
end show_busy

on clear_busy()
	-- Clear the progress indicator once the checks are finished
	set progress total steps to 0
	set progress description to ""
	set progress additional description to ""
end clear_busy

Start it when download_video() begins (currently line 1315), immediately after the handler opens:

	show_busy(localized string "Checking URL - please wait..." from table "MacYTDL")

Update the text at each slow stage — before the Deno check (around line 1486):

	show_busy(localized string "Checking Deno..." from table "MacYTDL")

and before the simulate run, section "Eighth, use simulated YTDL/yt-dlp run..." (currently line 1576):

	show_busy(localized string "Checking the video - please wait..." from table "MacYTDL")

Clear it before each display dialog inside download_video, before the Monitor dialog takes over, and immediately before end download_video (currently line 2793).

Three new strings would be needed for MacYTDL.strings: "Checking URL - please wait...", "Checking Deno...", "Checking the video - please wait...".

The handlers above compile and run under osacompile. They have not been tested inside MacYTDL itself, since editing the installed app would break its code signature.

Version: MacYTDL 1.31.2, macOS 26, Apple silicon.