Until now the repaint after fg was accidental: it only happened because
the suspend keybinding handler still had a flush pending on the UI
thread, and only if that flush happened to run after the SIGCONT
handler had re-engaged the screen. Now that flushes are skipped while
suspended, losing that race would leave the screen blank until the next
input event arrives, so schedule a redraw explicitly (#5309).
When suspending with ctrl+z, the suspend keybinding handler disengages
the screen and then sends SIGSTOP to the process group, so the UI
thread freezes at the return from kill(2) with the handler's follow-up
flush still pending. When fg continues the process, that pending flush
races the SIGCONT handler's Resume. If the flush wins, Show() draws
against the disengaged screen, whose cell buffer tcell has released to
0x0 while its width/height still hold the old size; drawCell() then
reports width 0 for the out-of-range cell, the draw loop's
'x += width - 1' never advances, and the UI thread spins forever while
holding the tcell screen lock. Resume in turn blocks forever on that
lock, so the screen never re-engages and no input is ever read again:
the hard stall of #5309, only recoverable by killing the process.
Guard both flush paths with the suspended flag. For the flag to
guarantee that the screen is engaged whenever it is false, Resume must
clear it only after re-engaging (it used to clear it before); Suspend
already sets it before disengaging. This also covers the pre-existing
unsynchronized suspended check in draw(), which is subsumed by the
guards and can go.
The regression test cannot use the demonstrate-then-fix pattern: on
unfixed code the flush goroutine spins holding the screen lock, which
deadlocks any subsequent screen call including the test cleanup's
Close().
When lazygit is suspended with ctrl+z and brought back with fg, nothing
deliberately triggers a redraw. The screen only repaints because the UI
thread happens to have a flush pending from the suspend keybinding, and
that flush races the SIGCONT handler's resume; when it loses in the
right way, the terminal shows a blank screen until the next input event
arrives (#5309).
