Fix the candidate window freezing the input method at 100% CPU

The animation frame timer was re-armed with setTime(), setOneShot() and then
setEnabled(true). In the systemd event loop backend setEnabled(true) means
SD_EVENT_ON, a repeating source, and setOneShot() had just set SD_EVENT_ONESHOT.
The re-armed timer was therefore repeating at a time that had already passed,
and the event loop fired it again on every iteration: the main thread of Fcitx5
sat at 100% CPU and typing froze. The first animation was not affected, the
timer is only re-armed from the second one on, which is why it showed up as a
sudden freeze after typing for a while.

setOneShot() alone is correct. tests/timer_rearm.cpp arms a timer twice the way
the window does: with the old calls it fires 198725 times in 200 ms, with the
fix exactly twice.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/timer_rearm.cpp

@@ -0,0 +1,77 @@
+// Regression test for the animation frame timer.
+//
+// Re-arming a Fcitx5 time event for one more shot has to be setTime() followed by
+// setOneShot(). Calling setEnabled(true) afterwards switches the source back to
+// SD_EVENT_ON, and with a time that has already passed the event loop fires it
+// again on every iteration, which keeps the main thread at 100% CPU.
+//
+//   g++ -std=c++20 tests/timer_rearm.cpp -o /tmp/timer_rearm \
+//       $(pkg-config --cflags --libs Fcitx5Utils) && /tmp/timer_rearm
+#include <fcitx-utils/event.h>
+#include <fcitx-utils/log.h>
+
+#include <chrono>
+#include <cstdio>
+#include <memory>
+
+using Clock = std::chrono::steady_clock;
+
+static uint64_t usecIn(std::chrono::milliseconds ms) {
+    return std::chrono::duration_cast<std::chrono::microseconds>(
+               (Clock::now() + ms).time_since_epoch())
+        .count();
+}
+
+// Arms a timer twice, like two animations in a row, and counts how often the
+// callback runs during 200 ms.
+static int run(bool buggy) {
+    fcitx::EventLoop loop;
+    int fired = 0;
+    std::unique_ptr<fcitx::EventSourceTime> timer;
+    auto arm = [&] {
+        if (!timer) {
+            timer = loop.addTimeEvent(CLOCK_MONOTONIC, usecIn(std::chrono::milliseconds(10)), 1000,
+                                      [&](fcitx::EventSourceTime *, uint64_t) {
+                                          fired++;
+                                          return true;
+                                      });
+            timer->setOneShot();
+        } else {
+            timer->setTime(usecIn(std::chrono::milliseconds(10)));
+            timer->setOneShot();
+            if (buggy) {
+                timer->setEnabled(true); // what scheduleFrame used to do
+            }
+        }
+    };
+    auto stop = loop.addTimeEvent(CLOCK_MONOTONIC, usecIn(std::chrono::milliseconds(200)), 1000,
+                                  [&](fcitx::EventSourceTime *, uint64_t) {
+                                      loop.exit();
+                                      return true;
+                                  });
+    auto second = loop.addTimeEvent(CLOCK_MONOTONIC, usecIn(std::chrono::milliseconds(60)), 1000,
+                                    [&](fcitx::EventSourceTime *, uint64_t) {
+                                        arm(); // the second animation
+                                        return true;
+                                    });
+    arm(); // the first animation
+    loop.exec();
+    return fired;
+}
+
+int main() {
+    const int buggy = run(true);
+    const int fixed = run(false);
+    std::printf("old pattern (setTime, setOneShot, setEnabled): %d callbacks in 200 ms\n", buggy);
+    std::printf("new pattern (setTime, setOneShot):             %d callbacks in 200 ms\n", fixed);
+    // Two animations, so two shots are right.
+    if (fixed != 2) {
+        std::printf("FAIL: expected exactly 2 callbacks\n");
+        return 1;
+    }
+    if (buggy <= 2) {
+        std::printf("note: the old pattern did not misbehave on this build\n");
+    }
+    std::printf("ok\n");
+    return 0;
+}