keyboard/printer: release modifiers before pressing new ones (fixes pikvm/pikvm#1549)

[mawadSur]

♻️ Reopens #224. Auto-closed when I accidentally deleted my fork; the commit (536ae6a) is unchanged, re-pushed from a fresh fork.

Problem

Fixes pikvm/pikvm#1549.

Pasting/typing text via the Web UI Text feature (POST /hid/print → text_to_evdev_keys) breaks the input when two consecutive characters need different modifiers. The classic repro on the Swedish layout is the sequence }":

• } = AltGr + 0
• " = Shift + 2

After this sequence, all following input produces wrong characters until a modifier key is tapped. The reporter confirmed it on ~20 layouts: cz, da, de, de-ch, es, fi, fo, fr-ca, fr-ch, hr, hu, is, it, lv, nl, no, pt, pt-br, sl, sv.

Root cause

In text_to_evdev_keys, the Shift modifier block was always handled before the AltGr block. When switching from an AltGr-character to a Shift-character, this pressed KEY_LEFTSHIFT before releasing KEY_RIGHTALT, so for one moment both Shift and AltGr were held at the same time — which selects a different symbol level on these layouts and corrupts subsequent input.

This matches the evtest capture in the issue exactly (LEFTSHIFT down emitted before RIGHTALT up):

RIGHTALT  ↓
KEY_0     ↓ ↑
LEFTSHIFT ↓     <-- pressed while AltGr still held
RIGHTALT  ↑     <-- released too late
KEY_2     ↓ ↑

Fix

Release the modifiers that are no longer needed before pressing the newly required ones, so Shift and AltGr are never held simultaneously across a key switch. Modifiers shared by consecutive characters are still kept pressed (the existing optimization is preserved).

Resulting order for }":

RIGHTALT  ↓
KEY_0     ↓ ↑
RIGHTALT  ↑     <-- released first
LEFTSHIFT ↓
KEY_2     ↓ ↑
LEFTSHIFT ↑

Tests

Adds testenv/tests/keyboard/test_printer.py:

• test_ok__printer_no_modifier_overlap — invariant (parametrized over several sequences): Shift and AltGr are never held at once, and everything is released at the end.
• test_ok__printer_release_before_press — asserts the exact corrected event order for }".
• test_ok__printer_keeps_shared_modifier_pressed — ensures a modifier shared by consecutive characters stays held.

Note: I could not run the Docker testenv (make tox) locally — it requires Linux (evdev/libxkbcommon). The change was validated against the lint configs and via a standalone simulation of the old vs. new ordering. Please run CI to confirm.

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