A personal, non-commercial segmented download manager for Windows x64, written from scratch in C++17 with Qt 6 Widgets for the user interface and the WinHTTP transport stack.

This project is a private learning exercise. It is not affiliated with, endorsed by, or derived from Tonec Inc. or Internet Download Manager (IDM). No third-party binary or decompiled source is redistributed here. The original installer is used only as a behavioral reference for feature research.

• What it does
• Evidence provenance
• Requirements
• Building
• Installer
• Running
• Chrome extension integration
• Source layout
• Download engine behavior
• Headless test harness (flextest)
• Local test server
• Credits
• Known limitations

• Adds downloads by direct URL, from the clipboard, or by drag-and-drop.
• Probes the server for byte-range support, then splits the transfer across up to N parallel connections, one per part file.
• Resumes interrupted or paused downloads, including across process restarts, by persisting part state to a sidecar file.
• Falls back gracefully to a single sequential stream when the server ignores Range (HTTP 200) or uses Transfer-Encoding: chunked.
• Verifies/repairs connections that end early and retries transient failures.
• Adopts a server-suggested name from Content-Disposition when the name was guessed from the URL.
• Minimizing or closing the window hides it to the notification area; downloads keep running in the background. The tray icon restores the window; use its Exit action (or File ▸ Exit / Ctrl+Q) to quit fully. Exiting with active downloads asks for confirmation first.
• Optional Chrome/Chromium integration: a browser download opens the IDM-style Download File Info dialog. If the desktop app is not running, Chrome keeps the download instead of losing it.
• Per-user settings under HKCU\Software\FLEXDownloadManager (connections, retries, timeouts, user agent, temp/save folders, proxy, speed limit, minimum part, remembered category folders and custom categories). On first run the app copies any existing settings from the old HKCU\Software\IDMClone key.

The main window follows the classic IDM layout and is built entirely with Qt Widgets:

• Menu bar - Tasks, File, Downloads, View, Help, Registration. The Downloads menu mirrors IDM: Pause All, Stop All, Delete All Completed, Find (Ctrl+F), Find Next (F3), Scheduler, Start queue / Stop queue submenus, Speed Limiter and Options. Right-clicking a row opens the per-item commands (Download Now, Stop Download, Redownload, Remove, Open file/folder, Copy URL).
• Toolbar - icon + caption buttons for Add URL, Resume, Stop, Stop All, Delete, Delete Completed, Options, Scheduler, Start Queue, Stop Queue and Tell a Friend, grouped with separators. Icons come from the embedded Fluent UI System Icons font (see Credits).
• Categories pane on the left — All Downloads (Compressed, Documents, Music, Programs, Video), Unfinished, Finished, Grabber projects, Queues. Selecting a category filters the list by file type or state; the pane can be hidden from its ✕ button or from View ▸ Categories.
• Download list — File Name, Q, Size, Status, Time left, Transfer rate, Last Try Date, Description.
• Status bar — item counters, aggregate speed and a progress bar.

A dark Qt stylesheet and palette reproduce the reference dark theme, including the dark window frame on Windows 10/11.

Dialogs: Add download, Download File Info (from Chrome), Options, About. The tray icon shows a balloon and a context menu; double-click restores the window. Minimizing or closing hides the window to the notification area while the app and its downloads keep running. View ▸ Language switches between English and العربية at runtime, including the RTL layout; View ▸ Arrange files exposes the table sorting options.

Feature research was performed with REA (rea-agents 6.7.0) driving Ghidra 12.1.4 as the native analysis provider.

Target binary (personal copy, kept outside version control):

The installer is a self-extracting archive of ~192 zlib streams starting at 0x17420. The embedded IDMan.exe (image base 0x400000, ~18.8k procedures) is MFC-based; class names such as CFilePart, ChunksList, CQueueTab, HTTPConnection, FTPSession and CM3UParser were used to confirm which capabilities to reproduce. Extracted strings (analysis/strings_all.txt, ~40k entries) corroborate the HTTP/range/timeout surface.

All clone code in src/ is original. Nothing was copied from the target.

• Windows 10/11 x64
• Visual Studio 2022+ (or VS 2026) with the Desktop development with C++ workload
• Qt 6.5 or newer with the Widgets module (tested with Qt 6.8.3, win64_msvc2022_64)
• CMake ≥ 3.21 and Ninja (both ship with the VS CMake component)
• Optional: Chrome/Chromium 102+ for the browser extension (the desktop app works without it)

build.bat assumes the VS install at C:\Program Files\Microsoft Visual Studio\18\Community and Qt at C:\Qt\6.8.3\msvc2022_64; edit VSROOT or set QTROOT if yours differs.

At runtime the Qt DLLs and the platforms\qwindows.dll plugin are required. build.bat runs windeployqt so the build folder is self-contained.

The download engine itself still uses only inbox Windows libraries (winhttp, shlwapi, shell32, ole32, user32, gdi32, advapi32).

cd idm-clone
cmd /c build.bat            :: Release (default)
cmd /c build.bat Debug      :: Debug

Outputs:

• build\FLEX Download Manager.exe — the GUI application
• build\flextest.exe — console engine harness (not shipped)

The build is warning-clean under /W4 /permissive- /utf-8 /EHsc.

The app and Chrome-extension icons are generated from FlexLOGO.svg. Regenerate both after changing the logo (requires Chrome/Chromium and Pillow; the normal build uses the checked-in assets):

python tools\make_icon.py

Use --no-extension-icons to update only src\app.ico.

installer/ contains a small, dependency-free Win32 setup program (static CRT, no Qt, no Inno Setup/NSIS) published under the ProSoft name. It embeds the whole application as one RCDATA payload, so the result is a single self-contained .exe.

cmd /c installer\build_setup.bat

The script expects build\FLEX Download Manager.exe (run build.bat first) and produces:

• dist\FLEX Download Manager 1.0 Setup.exe — setup + uninstaller + payload

Behaviour:

• Per-user install, no UAC: %LOCALAPPDATA%\Programs\FLEX Download Manager, with an uninstall entry under HKCU\Software\Microsoft\Windows\CurrentVersion\Uninstall\{7F1C2A54-9D3E-4B7A-9E64-2B9F1D6C0A31}. Everything is removed again by the uninstaller, including the shortcuts and the per-user files the setup created.
• Theme-matched UI using the application's colours (#1e252f, #252d39, #386ea8, ...), owner-drawn controls, dark DWM title bar.
• English and Arabic (RTL) interface, auto-detected from the Windows display language and switchable from the header.
• Desktop and Start menu shortcuts in a ProSoft program group.
• Direct streaming extraction from the embedded payload with real progress, no temporary archive and no external tools.
• Browser extension guide on the finish page: the extension is installed to browser-extension\ next to the application, and the page detects Chrome, Brave and Edge and walks the user through Load unpacked.
• Uninstaller (unins000.exe) built from the same sources. The UI uninstall copies itself to %TEMP% first so the application folder can be deleted completely.

Command line:

Code signing is optional; set PROSOFT_CERT, PROSOFT_CERT_ARGS and PROSOFT_SIGNTOOL before running the build script to sign the application, the uninstaller and the setup executable.

Chrome, Brave and Edge refuse to install an extension that is not published in their stores: since Chrome 33 the registry "external extension" method only accepts Chrome Web Store entries, and ExtensionInstallForcelist only works for store-hosted extensions (or domain/Azure-AD managed devices). Elevating the installer therefore does not make a store-less extension install silently.

The setup copies the unpacked extension next to the application and shows the four-step guide instead, which works on any machine without publishing anything. If the extension is later published to the Chrome Web Store and Edge Add-ons, the installer can be extended with a policy-based silent install by writing ExtensionInstallForcelist under HKLM\SOFTWARE\Policies\... — a step that does require administrator rights.

"build\FLEX Download Manager.exe"

Add a URL with Ctrl+N (or File ▸ Add URL…), pick a folder, and start it. Minimizing or closing the window hides it to the tray; double-click the icon to restore it, or right-click for Show main window / Exit. The app keeps running and downloads continue in the background. Exit (or Ctrl+Q) quits completely; if downloads are still running it asks for confirmation first.

The optional MV3 extension in chrome-extension/ forwards Chrome downloads to the running desktop app. When Chrome starts a download, the extension sends its URL, suggested file name, MIME type, total size and referrer to the app, and the app opens the IDM-style Download File Info dialog:

• URL (editable), Category with + for a custom category, Save As with ... for a folder picker, a remembered-path group for the selected category, Description, the file icon and size, and Download Later / Start Download / Cancel.
• The layout mirrors for Arabic (RTL) and follows the app's dark theme.
• Category folders remembered with the checkbox are stored per category under HKCU\Software\FLEXDownloadManager and applied to later downloads of that category.
• Custom categories added with + are persisted and appear in later dialogs.
• The size comes from Chrome's totalBytes when available; otherwise the app probes Content-Length with a Qt HEAD request.
• Browser-triggered downloads surface only that dialog: the main window is not shown or raised, and the dialog is placed on the screen with the pointer, kept above the browser and given the foreground so it is ready to type in. Closing it leaves the app in the notification area.

Automatic capture stops the browser's copy as soon as Chrome creates the download item and holds filename determination until the app answers. Chrome cannot reach the step that opens its own Save As dialog while that callback is pending, so the browser dialog no longer appears next to Download File Info even when Ask where to save each file before downloading is enabled. If the app is offline or rejects the URL, the paused download is resumed untouched and Chrome behaves normally. A ten-second safety timer resumes the download if the app never answers.

1. Start "build\FLEX Download Manager.exe".
2. Open chrome://extensions in Chrome/Chromium.
3. Enable Developer mode, choose Load unpacked, and select the chrome-extension folder (or the extracted dist\FLEXDownloadManager-extension.zip).
4. Pin FLEX Download Manager to the toolbar. The popup shows whether the desktop app is reachable and lets you turn automatic capture and the media panel on or off.

The setup program copies the extension to browser-extension\ inside the installation folder and repeats these steps on its finish page, with a button for every detected browser.

Branded Chrome 137+ removed the --load-extension command-line switch. Use Chrome for Testing/Chromium for scripted testing, or load the folder manually as above. The extension also adds a Download with FLEX Download Manager context-menu item for links, images, video and audio.

On pages with a visible <video> or <audio> element, the extension shows a small Download this video / Download this audio toolbar glued to the player's top edge and aligned with its right edge, with no gap between the strip and the frame (it drops just below the frame when the page top leaves no room). The panel appears when the media is detected, on hover, and on playback; it is rendered in a shadow root, so it does not inherit or alter the page's media controls. It offers:

• A single panel button. When the page exposes one video/audio file, clicking it sends that file to the desktop app and opens Download File Info.
• When more than one rendition is available, clicking the panel opens a drop-down list with the video name, every resolution, and the file size (when the server reports it). Clicking a resolution sends that exact stream to the desktop app and opens Download File Info for it.
• Qualities are detected from <source> elements, media attributes, URL resolution hints (1080p, 1920x1080, itag=137), and the media requests the service worker sees. Network capture is what makes the panel work on players that use MSE (blob: URLs): the <video> element has no downloadable URL, but the streams it fetched can still be offered. The list is sorted from highest to lowest resolution.
• On YouTube, Vimeo, TikTok, Instagram, X/Twitter, Facebook, Reddit, Twitch, SoundCloud and the other sites listed in content.js (YTDLP_SITES), the panel asks the desktop app for a quality list from yt-dlp as soon as the video is detected/played, and caches it per video. Every resolution the site publishes is listed with its approximate size, including 1080p/1440p/4K.
• ? help and X close buttons. Closing hides the panel for that media element until the page is reloaded.
• English and Arabic labels, following the extension UI language.

For those social sites the download runs through the bundled yt-dlp.exe instead of the HTTP engine: the app merges the separate video and audio tracks with ffmpeg when the chosen resolution needs it. ffmpeg is fetched once, on the first merged download (~115 MB, gyan.dev build), unpacked with Windows' tar.exe, and kept in %LOCALAPPDATA%\FLEXDownloadManager\ffmpeg\ffmpeg.exe. yt-dlp is looked up next to the executable, in its tools folder, in that per-user folder, or on PATH, so dropping in a newer build is enough to update it.

The panel works independently of the Capture Chrome downloads setting: it is an explicit user action. The popup has a separate Show media download panel toggle, on by default. The panel ignores tiny/hidden media, follows the frame on scroll and resize, and hides completely while the player is in fullscreen, coming back when the user leaves it.

Adaptive streams: HLS (.m3u8) and DASH (.mpd) manifests are detected but are not offered as direct downloads. The desktop engine downloads a single HTTP file; it does not fetch, decrypt or mux adaptive segments itself. On the supported social sites the panel delegates to yt-dlp, which handles those manifests; elsewhere, use the page's own player or a direct-file source.

Note: If the original Internet Download Manager is installed, its browser integration can capture a link before Chrome creates a download item. Disable IDM's browser integration (or close IDM) while testing this extension, otherwise both tools can compete for the same download.

If Chrome is set to Ask where to save each file before downloading (chrome://settings/downloads), Chrome opens its own Save As dialog before the extension can cancel the download, so both dialogs appear. Turn that option off to see only the FLEX Download Manager dialog. The extension popup has a shortcut to the Chrome download settings. As a one-off, Download with FLEX Download Manager in the context menu bypasses Chrome's download flow entirely.

Chrome does not expose this preference to extensions, and downloads.cancel() cannot close the already-open native Save As dialog. The setting is therefore a one-time browser configuration, not something the extension can toggle.

• The bridge binds only to 127.0.0.1, tries ports 48765–48769, and accepts requests only from chrome-extension:// origins. No native-messaging host or registry entry is installed.
• The extension asks for webRequest and all-site host access so the media panel can list streams that players fetch through MSE. Captured URLs stay in the service worker's memory per tab, are dropped when the tab closes or navigates, and are never sent anywhere except the local desktop app when you click one.
• The extension forwards the download first. Chrome's copy is canceled and erased only after the app acknowledges the request. If the app is not running, the Chrome download is left untouched.
• If a manual context-menu download cannot reach the app, the extension shows a notification instead of losing the link.
• The media panel sends explicit panel clicks straight to the local bridge; it never starts a Chrome download item, so it works even when automatic capture is off. HLS/DASH manifests are intentionally not offered.
• The popup's Capture Chrome downloads toggle is on by default; turning it off leaves all downloads in Chrome. Show media download panel is also on by default and can hide the overlay without disabling capture.
• The popup is localized in English and Arabic.

powershell -ExecutionPolicy Bypass -File tools\pack_chrome_extension.ps1

Outputs:

• dist\FLEXDownloadManager.crx — packed extension
• dist\FLEXDownloadManager-extension.zip — unpacked-folder zip for Load unpacked
• chrome-extension.pem — generated on the first pack; keep it private and reuse it to keep the extension ID stable.

Chrome may refuse to install a CRX that is not distributed through the Web Store; the unpacked folder or the zip is the reliable development path.

• Probe. A ranged GET for bytes 0-1 classifies the resource:
  • 206 → resumable; total size from Content-Range.
  • 200 → not resumable unless Accept-Ranges: bytes and not chunked; transfer runs as a single stream.
  • 416 → treated as complete/empty.
  • ≥400 → item enters the Error state with the HTTP status text.
• Segmentation. Total size is divided into parts of at least minPartBytes (default 1 MiB), capped at maxConnections (default 8).
• Part files. Each part is written to a temp file; on completion the parts are stitched into the destination and the temp directory is cleaned up.
• Resume. A binary sidecar (PersistedState + per-segment records) stores the URL, total size, and each part's start/end/written/done state, so a later run — even a new process — continues instead of restarting.
• Open-ended. When the length is unknown, one sequential segment is used and the openEnded flag disables range splitting.
• Retries. Transient failures retry up to maxRetries (default 5) with backoff; downloadedBytes accounting is corrected on retry so the speed and progress readouts stay honest.
• Cancellation. HttpClient::Abort() closes the WinHTTP handles to unblock a thread sitting in a read; the worker then observes stopRequested_.
• Redirects. Followed with a policy cap, and the final URL is captured for display and resume metadata.
• Notifications. The engine publishes progress through atomic counters on each DownloadItem; the Qt main window refreshes them on a 300 ms timer and only rebuilds rows when the visible set changes.

flextest <url> <output-path> [connections] [--pause-after MS] [--pause-only] [--cd-name]

• connections — parallel connections (default 8).
• --pause-after MS — stop after MS milliseconds, verify the Paused state, then resume.
• --pause-only — stop and exit, leaving part files for a separate resume run.
• --cd-name — treat the name as URL-derived so a Content-Disposition name replaces it (mimics the Add dialog with an empty name).

Examples:

:: plain segmented download (8 connections)
build\flextest.exe http://127.0.0.1:8765/payload.bin out\payload.bin 8

:: pause, then resume
build\flextest.exe http://127.0.0.1:8766/payload.bin out\payload.bin 8 --pause-after 400

:: cross-process resume
build\flextest.exe http://127.0.0.1:8766/payload.bin out\payload.bin 8 --pause-only
build\flextest.exe http://127.0.0.1:8766/payload.bin out\payload.bin 8

:: server ignores Range (HTTP 200) / chunked
build\flextest.exe http://127.0.0.1:8765/norange.bin out\norange.bin 8
build\flextest.exe http://127.0.0.1:8765/chunked.bin out\chunked.bin 8

:: Content-Disposition rename -> greeting.txt
build\flextest.exe http://127.0.0.1:8766/file.txt out\file.txt 4 --cd-name

A successful run prints complete: size=<n> path=<file> and the file's SHA-256 matches the expected payload hash.

python tools\test_server.py 8765
python tools\test_server.py 8766 --delay 0.05 --chunk 32768

Endpoints: /payload.bin (~3 MiB, range-capable), /norange.bin and /chunked.bin (512 KiB, deliberately non-splittable), and /file.txt (served with Content-Disposition: attachment; filename="greeting.txt"). All endpoints answer HEAD, so the Chrome dialog can probe Content-Length.

• Application logo: FlexLOGO.svg, supplied with the project.
• Toolbar icons: Microsoft Fluent UI System Icons, MIT licence. The build embeds a subset containing only the glyphs used by the application (src/fonts/FluentSystemIcons-Subset.ttf); the full licence text ships in src/fonts/LICENSE-Fluent.txt.

• HTTP/HTTPS only; no FTP (the original IDM supports FTP — out of scope here).
• Browser integration is Chrome/Chromium-only (MV3 extension). There is no Firefox/Edge-specific package. Outside the social sites handled by yt-dlp, the media panel only downloads direct HTTP(S) renditions; it does not fetch or mux HLS/DASH adaptive streams.
• CRX installs outside the Chrome Web Store may be blocked by Chrome; use Load unpacked for development.
• The Chrome dialog only appears while the desktop app is running. If it is offline, Chrome keeps the download and the extension sends a notification for manual context-menu captures.
• Part stitching is done in a single pass and rewrites the whole file; no sparse/in-place assembly.
• Proxy modes are direct, use system proxy (WinHTTP automatic proxy, which covers PAC/WPAD), and manual host:port. There is no per-site proxy rule list.
• Speed limit is enforced per connection, not globally, so the aggregate rate can reach connections × limit.
• Resume records are validated by size/URL only, not by an ETag or Last-Modified guard, so a changed remote file can still be resumed against.
• Drag-and-drop and clipboard add accept a single URL at a time.