hw_wallet/qt: reuse message dialog to fix lag between device prompts

[sashazykov]

Signing a transaction with many outputs on a Trezor is noticeably laggy: after I confirm one output on the device, there's a short pause before the next output shows up on the device screen.

What pointed me at Electrum's GUI rather than the device or the library: I have an Android signer (welldone-tech/satoshi-signer) built on the same trezorlib, and it has no such delay — confirming an output brings up the next one instantly. Same library, same device, so the bottleneck is on Electrum's host/UI side.

Symptom

When signing a tx with many outputs, each output confirmation on the device is preceded by a visible pause. On macOS you can see why: the "check your device" popup slides closed and a brand-new one slides open for every output, instead of staying put.

Root cause

QtHandlerBase.message_dialog() tore down the current dialog (clear_dialog() → QDialog.accept()) and built a fresh WindowModalDialog on every device button request. A device emits one button request per output, so signing a tx with N outputs rebuilds the dialog N times. A window-modal QDialog is rendered as an animated "sheet" on macOS, so the popup visibly slides closed and reopens on each request. That GUI churn also competes with the (single) hardware-comms thread for the GIL, adding latency between device prompts.

Fix

Reuse the already-open dialog and just update its label text when one is showing, instead of tearing it down and rebuilding. The window title and on_cancel handler are stable within a signing flow, so the only thing that changes between requests is the message text. If a new request comes in with a different on_cancel, we still rebuild (the existing dialog's cancel wiring would otherwise be wrong).

This also smooths Ledger's repeated progress messages (Signing transaction... (phase2, i/N)), which update the label text on each call.

Testing

Tested on macOS with a Trezor: signing a many-output transaction no longer slides the popup closed/open between outputs, and the lag between prompts is gone — outputs now appear back-to-back.

(I also have a small headless unit test for the dialog-reuse logic if you'd like the suite to start covering this — left out by default, since it would be the first QtWidgets-based test in the tree and pulls in a new QT_QPA_PLATFORM=offscreen setup.)

[sashazykov]

I have been testing this for a month alongside my other fixes and have not noticed any regressions. This fix may not seem like a big deal, but signing many outputs every day is common for businesses that process customer withdrawals and other high-volume transactions, and in that scenario it saves a lot of time.

[f321x]

Tested this on KDE/Fedora, for me there is no noticeable lag when the dialog gets rebuilt between the outputs.
It is probably due to this MacOS sheet sliding animation taking some time to slide in and out.
Nevertheless it does looks better if the dialog doesn't flicker that much either way.
Thanks.