* Ignore AltGr keystrokes in Ctrl+Alt keyboard shortcuts
Browsers report AltGr (right Alt on AZERTY/QWERTZ and most non-US
layouts, used to type @ # { } [ ] | \ and the euro sign) as
ctrlKey+altKey. The default keybinds map destructive actions to
Ctrl+Alt+<letter> (delete_session, new_session, incognito,
open_calendar), so a non-US user typing a special character could
silently fire them.
Guard the shortcut matcher, the editor keydown handler, and the rebind
capture with getModifierState('AltGraph'), which is true for AltGr but
false for a genuine left Ctrl+Alt. macOS is excluded: there the Option
key legitimately sets AltGraph and there is no AltGr/Ctrl+Alt collision
to guard against, so the guard would otherwise break Ctrl+Option /
Cmd+Option shortcuts (notably in Firefox).
The detection lives in one place — isAltGrEvent / IS_MAC in
static/js/platform.js — and all three call sites route through it, so the
guards can't drift apart.
The editor handler only skips the Ctrl+Alt chord block, so layout
shortcuts reachable via AltGr (e.g. [ ] brush size = AltGr+5/+8 on
AZERTY) keep working.
* Require Ctrl+Alt for the AltGr guard and consolidate keybind test marks
isAltGrEvent now also checks ctrlKey+altKey so it only suppresses the
"AltGr reported as Ctrl+Alt" collision; an event asserting AltGraph on
its own (a Linux ISO_Level3_Shift layout, a stray modifier) is left
alone. Pin it with test_isaltgr_false_when_altgraph_set_but_not_ctrl_alt.
Collapse the 12 per-test node skipif marks into one module-level
pytestmark, and note in platform.js why IS_MAC intentionally covers
iPad/iPhone and mirrors the isMac checks in calendar.js / sessions.js.
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